烯空气跟空气十分近似.Olefin gas closely approximates air.所有的主要难点都涌现在烯酮 ( 1 ) 的合成中.All the chief difficulties crop up in the synthesis of the enone ( 1 ).某碳氢化合物已知可能是烯烃或是烷烃.A hydrocarbon was known to be either an alkene or alkane....
Draw the floss backwards and forwards between the teeth.将牙线在齿间来回拉动。He controlled the car until it was pointing forwards again.他稳住车子，直到它又朝前开去。He stood a few moments, rocking back and forwards on his heels.他站了一会儿，踮着脚跟前后摇晃。...