氧指数OI是指在规定的条件下,材料在氧氮混合气流中进行有焰燃烧所需的最低氧浓度。以氧所占的比重百分数的数值来表示。()
参考答案:错
The letter____Tommy received from his company yesterday is very important.
A.who
B.where
C.what
D.that
______原则说明成本的增加并不一定会使土地价格增加。
A.最有效使用 B.需求与供给 C.报酬递增递减 D.预期收益