外科术后最常见的是（）
A．疼痛
B．发热
C．恶心、呕吐
D．腹胀
E．呃逆
参考答案：A
隧道长度在500mm及以下时，称为中隧道。
Which of the following is NOT a correct statement concerning covariance（）
A. It can only apply to two variables at a time.
B. A covariance of zero rules out any relationship.
C. It can be negative.
