题干
They ________ carefully, but they didn't ________ anything.
A：listened; listen
B：listened; hear
C：heard; listen
D：heard; hear
答案(点此获取答案解析）
B
They ________ carefully, but they didn't ________ anything.
A：listened; listen
B：listened; hear
C：heard; listen
D：heard; hear
是方程
的解，则