选A
这里是复合句,两个复合从句,第一个是 we could have learned__when we were at high school整个从句做宾语,作为宾语从句,另一个,__when we were at high school这个做learn的宾语从句,从句中的语序必须是陈述句语序,所以排除D,D是疑问句语序.you did 后面需要一个宾语,所以要what,当主谓宾都存在的时候才能用that,所以排除了B和C