I_____frustrated when I wasn‘t sure of the coorect answer A.used to B.used to be C.use to be
问题描述：
I_____frustrated when I wasn‘t sure of the coorect answer A.used to B.used to be C.use to be
答
B used to be 过去常常...
句意是：
过去,当我不确定哪个是正确答案时我就很泄气.