若{an}是等差数列，则a6＜a7是a6＜a8的（ ）
A．充分而不必要条件
B．必要而不充分条件
C．充分必要条件
D．既不充分也不必要条件
由题意，设等差数列的公差为d，则若a6＜a7，则d＞0，∴a6-a8=-2d＜0，∴a6＜a8∴a6＜a7是a6＜a8的充分条件；
若a6＜a8，则d＞0，∴a6-a7=-2d＜0，∴a6＜a7，∴a6＜a7是a6＜a8的必要条件；
∴a6＜a7是a6＜a8的充要条件；
故选C．
| 完形填空。 | |||
| There is a big zoo in our city. There are lots of 1 in it. The elephant is very big. We all like to see it, 2 it is very cute. The tiger comes from the Changbai Mountain of 3 .It walks about in the cage(笼子).The lion’s room is 4 the tiger’s room. The panda in the zoo is a little shy. It is very interesting. It eats 5 The giraffe is very 6 .It is very funny. It comes from Africa. We all 7 animals because they are 8 friends. Some boys and girls, men and women often come to the zoo and see 9 on 10 . | |||
| ( )1. A. people ( )2. A. and ( )3. A. China ( )4. A. in ( )5. A. trees ( )6. A. short ( )7. A. like ( )8. A. we ( )9. A. they ( )10. A. Sundays | B. dogs B. but B. America B. under B. bamboo B. high B. eat B. our B. them B. schooldays | C. monkeys C. because C. Japan C. next to C. vegetables C. tall C. beat(打) C. us C. their's C. workdays | D. animals D. why D. England D. above D. rice D. fat D. catch D. our's D. their D. birthdays |