They have gone to Beijing for a meeting. They will be back ________.
A:after a few weeks
B:in a few week's time
C:in a few weeks' time
D:for a few weeks
秋天是一个秋高气爽的季节。秋天的树的叶子变黄了。秋天人们穿上了秋装。( )
①在烷烃分子中,所有的化学键都是单键
②烷烃中除甲烷外,很多都能使酸性KMnO4溶液的紫色褪去
③分子通式为CnH2n+2的烃不一定是烷烃
④所有的烷烃在光照条件下都能与氯气发生取代反应
⑤光照条件下,乙烷通入溴水中,可使溴水褪色.