Which one is the most expensive mask?
A. the gas mask
B. the surgical mask
C. the silk mask
D. the gold mask
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What should you do during the epidemic situation?
A. Washing your hands frequently
B. Stay indoor as you can
C. Wearing masks if you need to go out
D. Eat a lot.
力法典型方程中自由项为 ( ) 。
A. 恒为正
B. 可正可负
C. 可为零
力法典型方程中付系数δii表示基本体系由Xi=1产生的Xi方向上的位移。( )
A. 对
B. 错
Look at the pictures and match them with the phrases listed below.
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