引蒸汽前，应确认蒸汽管线上的疏水器（）。
A、已隔离
B、前切断阀已打开
C、后切断阀已打开
D、前、后切断阀和副线均已打开
参考答案：D
What _____the forests of the USA in the past 30 years? [ ]
A. has happened to
B. is happened to
C. has happened at
D. is happening
有以下程序： #include ＜stdio.h＞ int fun() { static int x=1; *x=2; return x; } main() { int i, s=1; for(i=1; i＜=3; i++)s* =fun(); printf("%d\n", s); } 程序运行后的输出结果是______。
A．10
B．30
C．0
D．64
