Shall Mary come to the concert with us? -Not she has finished her paper. A.since B.once C.when D.unless 答案:D 解析: 由句意得知,补全句子应该是Mary does not come to the concert with us unless she has finished her paper.
把一定质量的a、b、c、d四种物质放入一密闭容器中,在一定条件下反应一段时间后,得反应后各物质的质量如下。下列说法中正确的是( ) 物质 a b c d 反应前的质量/g 6.4 3.2 4.0 2.8 反应后的质量/g 5.2 X 7.2 2.8 A.a和c是反应物 B.d一定是催化剂 C.X=2.0g D.该反应是化合反应