1.B 2。 D 3。 C 4 explosive ... B 5 D 6。 C 7。 C 8。 A nine. A 10。 B

1 1. yellow;

12. 1;

13.8.；

14.20;

15.7n+8，50；

16.7×[3-(-3)-7];

17.( 1)-0.7;

19.50， 1+3+5+7+……+(2n+ 1)= N2

20.( 1) The money saved by buying two boxes of eggs by customer B is 2+( 14- 12)=4 yuan, so the two boxes of eggs bought by customer B are not cost-effective.

(2) 10 (unit)