某有机物的分子式为C200H202，已知该分子中叁键与双键数之比为2：1，则其双键数目为[ ]
A．50
B．40
C．30
D．20
答案：D
--- Journalists have to work very quickly in order to_______.
--- I know. I like that. It’s very challenging.
A．meet their deadlines
B．earn a high salary
C．avoid being fired
D．make both ends meet
减小车刀的主偏角，会使背向力Fp减小，进给力Ff增大。( )
