-48V电源线采用（）色电缆线。
A、黑
B、红
C、绿
D、蓝
参考答案：D
点（5，9）与点（x，y）关于原点对称，则x+y=______．
Let us meet at the gate of the school, _____? [ ]
A. shall we
B. don't we
C. will you
D. do you
