By the time he was ten, Edison ________

试题详情及答案解析

By the time he was ten, Edison _________ experiments in chemistry.

A．had already done	B．already had done
C．was already doing	D．already did

答案：A

试题分析：考查时态。根据句中时间状语By the time he was ten到他十岁的时候，可知用过去完成时。句意：爱迪生十岁时,已经开始做化学实验了。故A正确。
考点：考查时态