B

    Have you ever wondered why the roots of the plants always know which way to grow—into the soil but not above it? Some British scientists have recently solved this mystery.    

      It turns out that roots have special hairs that tie them into the soil and help them grow their way past obstacles, a team at the John Inners Center in Norwich reports in the February 29 issue of Journal Science.

    "The key is in the fuzzy(有绒毛的) coat of hairs on the roots of plants," says professor Liam Dolan. "We have found a growth control system that enables these hairs to find their way and to become longer when their path is clear."

    Root hairs explore the soil in much the same way a person would feel their way in the dark. If they come across an obstacle, they make their way around until they can continue growing in an opening. I the meantime, the plant is held in place as the hairs grip the soil.

    The hairs are guided by a clever chemical trick. A protein(蛋白质) at the tip of the root hairs called RHD2 helps them to take calcium(钙) from the soil. Calcium makes the hairs grow, and produce more RHD2, and take more calcium.

    But when an obstacle blocks the hair's path, or the hair reaches the surface of the soil, the cycle is broken and growth starts in another direction.

    This system gives plants the flexibility to explore a complex environment and to live in even the most unpromising soils, says Dolan.

    In poor soils such as in parts of Australia and Africa, native plants have adapted by producing enormous numbers of root hairs. A better understanding of this adaptation will allow scientists to develop hairy rooted crops that can grow in unfriendly environments.

    According to Dolan, "Research in the John Inners Center is taking a breeding approach(育种方法) to increase hair length in wheat but it will be some time before new cultivars(栽培变种) are developed."

    Traveling for business or pleasure can easily damage your health. It's easy to fail into the trap of high caloric meals and little exercise.____

    Keep a dictionary on hand

    If you are traveling somewhere with a different language, pack a dictionary and keep it close to help you negotiate(商议)the menu and make healthy food choices.____Look up the words that you don't understand to help you learn about cooking methods and ingredients(材料)so that you can choose healthy food.

    ____

    The more you can research before your trip, the easier it will be to keep a healthy lifestyle while traveling. To stay on track with your food, try to find a nearby grocery store or healthy food store, look up a couple of well rated restaurants that offer some healthy choices, and try to stay somewhere with a fridge or kitchen.

    Walk every day.

    Exploring your destination(目的地)on foot is an amazing way to take some exercise while traveling. Walk through the nature to enjoy its beautiful scenery, listen to the sound of nature and take some wonderful pictures.____

    Rent a bike.

    Cover more ground while burning calories by renting a bike. Many major cities have bike-sharing programs, or you could contact a local bike store to ask about bike rentals.____If you have an open schedule, you could even research bike tours, which are a great way to cover a long distance in a short period of time.

A. Avoid junk food and deep fried food.

B. Research the food and store some upon arrival.

C. Take a bike map of the city, research day trips and get exploring.

D. If you are exercising while traveling, make sure to take in extra water.

E. Here are some healthy travel tips which will keep you in good health.

F. You can't avoid deep fried food if you don't know that they're in there!

G. This is also a great way to find hidden attractions that aren't in the travel guides.

无声手枪是在____ 处减弱噪声；放鞭炮时，用手使劲堵住耳孔，这是在____ 处减弱噪声；在居民区和马路旁植树造林，这是在____ 减弱噪声．