a_____expression on their face
第一个空是用puzzling还是puzzled
The students were puzzled over the puzzling problem, listening to the teacher with a puzzled expression on their face.
-ed 形容词表示 某人的感觉如何
-ing 形容词表示“令人感觉如何”“使人感觉如何”
此句的含义是:
学生们对这道让他们迷惑不解的问题很困惑,一脸困惑地聆听老师的讲解。