重点讲解：
1. prop up 支撑；维持；
例句：On the Stock Exchange, aggressive buying propped the market up.
在证券交易所，市场在大量买单的推动下有所上扬。
2. block off 封闭，封住（门、窗或通道）；
例句：Heavy snow has blocked off traffic on the motorways.
大雪阻碍了高速公路上的交通。
3. in advance 提前；事先；
例句：As long as you've paid in advance we won't charge you for delivery.
只要你预先付款，我们就不收你送货费。
4. give away 赠送；捐赠；
例句：We have six copies of the book to give away.
这本书我们要赠送6本出去。
5. as of 在…时；从…起；自…始；
例句：As of now we don't know much about Mars.
目前我们对火星还知之甚少。
6. stand at 处于（某一水平）；
例句：Your temperature stands at 100°F.
测得你的体温为100华氏度。
