— Mrs.Smith,please teach me how to draw on the computer today.

—Oh,no. ____.You have to get familiar with the keyboard first.

A．It is never too old to learn

B．Think twice before you do

C．Learn to walk before you run

D．One today is worth two tomorrows

《继电保护和安全自动装置技术规程》规定，来自电压互感器二次的四根开关场引出线中的零线和电压互感器三次的两根开关场引出线中的N线必须分开，（）。