He kept yawning in class this morning. That is he stayed up late doing his homework last night.
A. why
B. how
C. because
D. whether
网友回答
A. why
B. how
(答案→)C. because
D. whether
解析：句意：今天早晨上课他一直在打哈欠。那是因为他昨晚熬夜到很晚写家庭作业的缘故。通过分析句子结构，可知这个句子中包含一个表语从句，且表示“因为”，这样的意思，所以用because，选C。