题干

下列关于生物个体发育的叙述,哪些是家蚕和青蛙都具备的特点(  )

①发育过程在水中进行  ②发育的起点是受精卵  ③发育过程有蛹期  ④幼体和成体有很大差异.

A:①④

B:②③

C:①③

D:②④

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D

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    New York is one of the last large American cities to have some of its policemen on horseback. The New York police have 170 horses working in certain parts of the city. The horses are expensive to feed, but it is even more expensive to look after them. Because the horses must walk on the streets, they need special horseshoes. In fact, they need more than 8,000 of them each year. Every police horse in New York gets new shoes every month. Keeping these shoes in good repair is the job of six blacksmiths. There are only about thirty-five of these blacksmiths in the whole United States.

    The cost of shoeing a horse is between twenty dollars and thirty-five dollars, and it takes a good blacksmith two or three hours to do the job.

    A blacksmith's job is not an easy one. He must be able to shape a shoe from a piece of metal and then fit it to the horse's foot. The blacksmith must bend over all the time when he is fitting the shoe and must hold the weight of the horse's leg while he works. Clearly, a blacksmith must be very strong. But even more important, he must be able to deal with horses---for before the blacksmith can begin his work, he has to get the horse to lift its leg.

   One of the blacksmiths in New York is James Corbin. He came to the country from Ireland in 1948. He not only makes horseshoes for the police but also works for a group of horse owners near the city. Corbin became interested in blacksmithing because his father did it, and, as he puts it, “It's a good way to make a living.”