1.Mr.Green has four English books while Mr.Green has six English books.
知识点：while 表示前后对比.
2.Steve is the youngest boy in his class.
形容词比较级+than any other = the +形容词最高级
3.They bought the cheapet bike.
Which bike did they buy?
which 用于有比较、有分类时的提问,此时不可用what替换.
what侧重于问事物的性状