.Don’t make jokes in America. Even in experienced hands, a joke can be a dangerous thing. I came to this ______ recently while passing through Customs and Immigration in Boston..

A．decision

B．position

C．conclusion

D．illusion

安装管道法兰和阀门螺丝时，用手指触摸，找正法兰和阀门的螺纹孔，然后紧固螺丝