（）是学科，是构成学习领域的基本单位。
参考答案：科目
设集合A={a，b}，B={0，1}，则从A到B的映射共有（ ）
A．2 个
B．4个
C．6个
D．8个
—What were you doing at this time yesterday?
—We _______ in the classroom.
A．read
B．were reading
C．are reading
D．was reading
