矿井防治水工作的原则是什么？
参考答案：矿井防治水工作坚持“预测预报，有疑必探，先探后掘，先治后采”的十六字原则。
确定表面传热系数六的方法有( )。 ①类比法 ②实验法 ③分析法
A．①②③
B．①②
C．①③
D．②③
A four-character password consists of one letter of the alphabet and three different digits between 0 and 9, inclusive. The letter must appear as the second or third character of the password. How many different passwords are possible
A．5,040
B．18,720
C．26,000
D．37,440
E．(E) 52,000
