I forgot to return the book to you yesterday. So I（）today 选择正确的 A might do it B. must do it C. had to do it D. must have to do it

在这个句子中，最合适的答案是A. might do it。"might" 表示可能性，意为“也许我会在今天把书还给你”。这个选项更符合实际情况，而其他选项(B. must do it, C. had to do it, D. must have to do it)都强调了一种必须完成的行为，这在原句的语境下并不合适。