题目

如图所示，质量为m的球置于斜面上，被一个竖直挡板挡住、现用一个力F拉斜面，使斜面在水平面上做加速度为a的匀加速直线运动，忽略一切摩擦，以下说法中正确的是（ ）  A． 若加速度足够小，竖直挡板对球的弹力可能为零   B． 若加速度足够大，斜面对球的弹力可能为零   C． 斜面和挡板对球的弹力的合力等于ma   D． 斜面对球的弹力不仅有，而且是一个定值 Lisa has been to Antarctica (南极洲). Here is a page from her.Tuesday December 2We planned to go to Rothera that morning. We’d be staying there for the next two weeks. Because Antarctica is the windiest place on earth, sometimes you can’t fly at all.We had a nervous wait over breakfast to find out if we’d be leaving that day. People have to stay in Stanley for weeks while the pilots wait for good weather.It turned clear at 9: 30 and we took off at 10: 30 on a little red plane called a Dash-7. But even when we were in the air, there was still a chance we wouldn’t be able to fly the whole day.Wednesday December 3After waking up in the Antarctica for the first time today, I can understand why everybody who comes here falls in love with the place. It is really beautiful.We’re staying at Rothera Survey base with mountains of ice all around. It’s about minus 2℃ today, which for me is very cold, but the regulars here are often seen walking around in T-shirts!Thursday December 4I woke up to another beautiful sunny day here in the Antarctica. I’m told it’s a bit colder today, about minus 5℃, but it’s not very windy so it feels warmer. Those of us who are new to the base have to do a special training course before we're allowed to go off the base to other stations or to go snowboarding over the nearby hill.Saturday December 6Not a cloud in the sky and it’s warm enough to sit outside (in a jacket).The most amazing thing about this place is how the scenery changes every day. At first I thought I was going mad. I’d step outside in the morning and think, “I’m sure that big mountain of ice wasn’t there yesterday.” It’s because the sea ice is always moving—slowly thankfully.【1】Lisa sat outside wearing a jacket on .A.TuesdayB.WednesdayC.ThursdayD.Saturday【2】 could stop Lisa from leaving Stanley for Rothera.A.Strong windsB.Thick cloudsC.Mountains of iceD.Low temperatures【3】The underlined word “go off” most probably means .A.leaveB.changeC.guardD.watch【4】Lisa felt during her stay in the Antarctica.A.afraidB.nervousC.amazedD.bored【5】What left Lisa a deep impression about Rothera was that .A.people walked around in T-shirtsB.there was a special training courseC.the weather was sometimes warmD.the scenery changed every day【6】From the article we can know this is a page from Lisa’s .A.notebooksB.diariesC.homeworkD.books