题目

在通常情况下空气中各种成分的含量保持相对稳定．其中体积分数约为21%的气体是（ ） A．N2   B．O2   C．CO2  D．H2O 答案：【考点】空气的成分及各成分的体积分数． 【专题】空气与水． 【分析】根据空气中各成分的体积分数进行分析判断． 【解答】解：空气的成分按体积计算，大约是：氮气占78%、氧气占21%、稀有气体占0.94%、二氧化碳占0.03%、其它气体和杂质占0.03%． A、N2占78%，故选项不符合题意． B、O2占21%，故选项In the US, Mother's Day is a holiday on the second Sunday in May.It is a day when children give their _________cards, presents and flowers.One of the best ways to celebrate Mother's Day is to give your mother a day off.Let her have a good rest while other members of the family do the _________.Many families begin Mother's Day with _________in bed.Usually Dad and the children will let Mum__________ late as they go into the kitchen(厨房) and get ready_________ her favorite meal.A Mother's Day breakfast can consist of anything your mum likes.After the food is cooked, they keep everything nice on a plate. They don't forget to put a bottle __________ only one flower.It's spring here; the children can pick the nicest__________ from the garden outside.When everything is ready, Dad and children carefully carry the plate and Mum's favorite books and newspapers up to her bedroom.Cards and small presents from the children can be put on the plate_________it is given to Mum in bed.Many families take Mum out to her favorite restaurant for a meal.It is a good day to let your mum ________ and let her see what a wonderful_________she has.【1】A. mothers B. parents C. teachers D. friends【2】A. housework B. washing C. work D. shopping【3】A. breakfast B. lunch C. supper D. dinner【4】A. eat B. sleep C. wash D. cook【5】A. to B. for C. with D. by【6】A. in B. on C. with D. of【7】A. plate B. flowe r C. bottle D. food【8】A. after B. until C. if D. before【9】A. sleep B. eat C. cook D. rest【10】A. family B. job C. restaurant D. flower