【单选题】The cookies each _____________ letters. [ ]
【简答题】Are you going downtown this afternoon? I am going to have these letters mail . A B C D ____________
【单选题】下列关于阿司匹林的描述,不正确的是( )
B.
其解热、镇痛、抗风湿作用与其抑制PG生物合成有关
【单选题】下列说法中错误的是( ) ①化学性质相似的有机物是同系物 ②分子组成相差一个或几个CH 2 原子团的有机物是同系物 ③若烃中碳、氢元素的质量百分比相同,它必定是同系物 ④分子中含有碳、氢元素的化合物是烃类.
【单选题】I have four letters to write today and I have finished ______.
【单选题】You ______ all morning, but you ______ three letters!
A.
have been typing, have only done
C.
had been typing, have only done
D.
have typed, have only done