What --- is the best advice for learning English见下：答案为什么选B

do you think是插入语
在一个问句的疑问词中插入这个表示询问你的想法
比如What is her job?她做什么工作
插入do you think就成了What do you think is her job?你认为她做什么工作?
当然do you think变成过去式就成了did you think,问的是你过去的想法,原问句的谓语动词也要变成过去式.以你的例句为例,如果用did you think,就要变成What did you think was the best advice for learning English?