题目

换用高倍镜观察洋葱表皮细胞时，错误的操作是（ ） A . 调节粗准焦螺旋 B . 调节细准焦螺旋 C . 用凹面镜 D . 转动转换器 答案：AA good many trees ____ in this part of the country by the end of last year. [　　] A．have been planted B．had been planted C．would be planted D．were planted