期货期权买卖的双方都需支付保证金。 ( )
参考答案:错
Class 2 will go for a picnic. They made _______ 6 o'clock to meet at the foot of Mt. Tai. [ ]
A. it
B. that
C. them
D. /
焊前清除坡口边缘的目的是()。
A.增加熔深
B.减少焊缝金属中氢含量
C.防止焊缝产生咬边和夹渣
D.防止热裂纹