题目

已知a，b是两条异面直线，A，B∈a，C，D∈b，AC⊥b，BD⊥b且AB＝2，CD＝1，则直线a，b所成的角为( ) A．30°                              B．60° C．90°                              D．45° 答案：B 解析：因为， 所以＝1. cos＝⇒＝60°，故选B． What a careful boy you are! Thank youIn fact,Tom does everything ________ than me [　　] A． more carefully B． more careful C． much careful