清政府特许的统一经济管理贸易机构是（   ）

已知反比例函数y= $\frac{k}{x}$ （k≠0）的图像经过点M（﹣2，2），则k的值是（   ）

    My name is Joe. I have two friends Bob and Paul. We play sports and eat lunch together every day. We like hamburgers for lunch.

    Paul and I like French fries, but Bob doesn't. Bob and Paul like ice cream, but I don't. Paul has a great sports collection. He has 20 tennis rackets, 15 baseballs and 11 tennis. But he doesn't play soccer. Bob and I have a small sports collection. But we both have a soccer.

由于国际原煤价格上涨，少数不法商人把一种黑色石头掺在优质煤中高价出售，客户为了不上当受骗，辨别煤中是否掺杂的最恰当方法是检测下面哪一个物理量（    ）