小兰看一本故事书，第一天看了 $\frac{1}{6}$ ，第二天看了42页，这时已看的与未看的页数之比是2：3．这本书共有多少页？

     We make traffic rules to keep order on the roads. These rules also help keep us 1  on the roads.
People have to  2  many traffic rules. In China, you must walk  3  the sidewalk or on the side of the road. Look carefully (仔细地) before you   4  the street.
      If you are riding a bike, don't ride in the middle (中间) of the roads or run through (通过) the road when the traffic light is   5 . When you ride a bike  6  a friend, don't look around and talk.
     If you drive, you must slow down at the traffic lights. You must always  7  a seat belt安全带). Without (没有) belts, 8 the driver and the other people in the car may hurt themselves when the car stops suddenly (突然地).
     When you take a bus, you must be 9 . Get on or get off a bus only when it comes to a  10  When it's full (满的), you shouldn't get on it or you may fall down.

AaBb 1AB∶1Ab∶1aB∶1ab 配子间16种结合方式 子代中有9种基因型 4种表现型(9∶3∶3∶1)