单项选择题
A．are necessary for students to study systems B．require people to know how to measure C．tell one what is needed to learn to live there D．help one learn measure words used in that country
A．are easy to forget B．are understood more deeply C．are difficult to remember D．are hard to comprehend
A．they memorize them consciously B．they study them directly C．they do something they enjoy D．they listen to popular songs