由X和Y两元素组成的两种化合物甲和乙，甲的化学式为XY₂，其中X的质量分数为50%．若乙的化学式为XY₃，则乙分子中X的质量分数为（　　）

A．20%

B．30%

C．40%

D．75%

根据中文提示，写出正确的单词。

Uncle John is from_____小题1:____(澳大利亚). But he can   小题2:   (说) English, Chinese and Japanese.  He     小题3:   (教)English in a school of Shanghai. Uncle John likes to  小题4:   （锻炼）every day. He thinks it’s    小题5:  （重要的）to keep healthy. He usually rides his    小题6:   （自行车）to work. He ___小题7:_____（也）likes to make dinner for his three children. He always says that delicious food makes    小题8:  (我们)happy. He keeps a    小题9:   （宠物） dog, and everyone thinks his dog is lovely and     小题10:  （聪明的）. It can sing and dance. It can walk on two legs. I want to visit Uncle John this summer vacation.