以下制冷剂中，属于天然制冷剂的是（）。
A.R744
B.R22
C.R134a
D.R502
参考答案：A
The Tibetans have lived in the Himalayas for centuries and have adjusted to the conditions _______.[ ]
A. at such great height
B. at such a great height
C. on such great height
D. on such a great height
已知D（X）=4，D（Y）=9，Cov（X+Y）=2，则D（3X-2Y）等于（）．
A.96
B.72
C.48
D.36
