.Mary told me that she had been interested in ______I had told her.A. all that B. which C. that D. all what为什么选A

.Mary told me that she had been interested in ______I had told her.
A. all that B. which C. that D. all what
为什么选A

all 后面that可以看成是一个关系代词,引导定语从句,先行词就是all