设二维随机变量(X，Y)的联合密度函数为
，则k为( )
参考答案：C
解析：[详解] [*] 得k=6，选(C)．
With a lot of difficult problems _______, the manager felt worried all the time.
A．settled
B．to be settled
C．settling
D．to settle
什么是锅炉汽温的静态特性？
，则k为( )
，则k为( )