The mother was nervous because she had not her son who was away at a boarding school.
A. heard from
B. heard of
C. heard about
D. heard out
网友回答
(答案→)A. heard from
B. heard of
C. heard about
D. heard out
解析：短语辨析。A收到sb的来信；B=C听说；D听完；根据句意可知A正确。句意：那位母亲很紧张，因为她很久没有收到在住宿学生学习的儿子的来信。