三角形三边之比为3∶5∶7，与它相似的三角形最长边是21cm，则最短边是[ ]
A.6cm
B.9cm
C.12cm
D.15cm
答案：B
Read and write. (根据回答, 写出问句)
1. A: ___________________________?
B: I'm a policeman.
2. A: ___________________________?
B: I go home at 5:00 in the afternoon.
3. A: ___________________________?
B: I usually watch TV and go shopping on the weekend.
4. A: ___________________________?
B: I like winter best.
5. A:____________________________?
B: Because I can swim in the river.
试分析电力系统中性点各种运行方式特点及适用范围。
