一般现在时与现在进行时的用法 — 英语选择题
题目
选择最佳答案：Emma is a volunteer at the city library. She usually ______ (help) visitors find books on Saturdays, but she can't do it this Saturday because she ______ (prepare) for an important exam at home right now.
A. helps; is preparing
B. is helping; prepares
C. helped; prepared
D. helps; prepares