题目

Tina learnt to ride a bike by herself, but her twin sister _____by their father. A. were taught       B. taught          C. is taught               D. was taught 答案：D用一个网罩将一片草场罩起来，草可能会被虫子吃光了，其主要原因是A．罩住后虫子的天敌进不去，虫子的数量迅速增加，把草吃光了B．罩住后阳光能进去，植物不能生长而很快被吃光了C．网罩内还有其他的动物帮助吃草D．罩住后虫子迅速繁殖而把草吃光了