制冷剂在蒸发器中通过何种变化，产生空调器（冷气机）之冷却效果（）
A.液化
B.固化
C.升华
D.气化
参考答案：D
用所给单词的适当形式填空。
1. —What's the date today?
—It's the______ (twenty) of May.
2. Thank you for ______ (invite) me to your party. But I can't accept your______ (invite)
because I have a lot of work to do.
3. Please keep quiet. He______ (try) the piano.
4. This is my______ (brother) new shirt.
5. He has much work______ (finish).
GPRS网络子系统包括（）以及固定Internet网络上已经存在的DNS、RADIUS、BG等设备。
A、服务GPRS支持节点SGSN
B、网关GPRS支持节点GGSN
C、计费网关
D、PCU
