①两个氮分子____；②3个钙原子____；③4个硫酸根离子____；④水中氢元素的化合价为+1价____；⑤地壳中含量最多的金属元素____；⑥氧的原子结构示意图____；⑦核电荷数为12的金属离子____；⑧保持氢气化学性质的最小粒子____．

   Do you have the ability to live cheaply? Now here's a true story of an American girl，Sarah．1Sarah left university，she began to live in New York City on her own．She 2 a job in a publishing house，but the salary(薪水)was 3 $30，000 a year．Her parents thought her life would be hard，but Sarah 4live on her salary and still saved $5，000 in a year．How was that 5 in one of the most expensive cities in the world?
   Cheap living 6starts with keeping the big cost small．For most people，that 7housing．So Sarah chose to share a flat with three other friends．Her next biggest cost was  8．When she ate out，she went to cheap restaurants．9 she bought a $9．99 whole chicken at a local restaurant．She then10the chicken bones(骨头)home and made soup out of them．Nowadays young people often 11 a lot of money on entertainment(娱乐)．But Sarah said，"I enjoy walking 12 in New York City．I love going to museums and parks．" Did Sarah feel poor13 cheap living? Not really．She even managed to take two trips，one to the Netherlands，14to Portland．
   “Don't think of saving money as something15．It's a kind of game，"said Sarah． 

若函数e^xf（x）（e=2.71828…是自然对数的底数）在f（x）的定义域上单调递增，则称f（x）具有M性质，下列函数中具有M性质的是（　　）