Simon thought his computer was broken _______ his little brother pointed out that he had forgotten to turn it on

A.until

B.since

C.after

D.because

答案及解析： A

解析：

本题考查连词。根据句意他一直以为电脑坏了，直到他弟弟告诉他忘记开机了；

not...until...意为“直到……才”。