回采速度慢、采区尺寸过大容易发生自燃。
参考答案：对
Read and complete.
A: What 1. _ you do yesterday?
B: I 2. to see 3. .
A: What's the 4._ ?
B: I 5. a toothache.
A: 6. do you 7. now?
B: I feel better now. What 8.____ you 9. _ yesterday?
A: I 10. fishing with my friends.
PN型泵是（）离心砂泵
A、卧式多级悬臂式
B、卧式单级悬臂式
C、立式
