设f(u)具有二阶连续导数，且
参考答案：由于 [*] 故[*]
解析：[考点提示] 多元复合函数求偏导．
When it comes to reducing stress, enough sleep and physical exercise can help, _____. [ ]
A. so does proper nutrition
B. so proper nutrition does
C. so proper nutrition can
D. so can proper nutrition
引起老年人胸痛的常见原因是（）
A.胸膜炎
B.肺炎
C.心绞痛
D.风湿性心脏病
E.自发性气胸
