问题
选择题
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
答案
答案：C
题目分析：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。