A.a_n=2n-1，d=-2                       B.a_n=2n-1，d=2

C.a_n=-2n+1，d=-2                     D.a_n=-2n+1，d=2

A.a₁＞0且d＞0                                               B.a₁＞0且d＜0

C.a₁＜0且d＜0                                               D.a₁＜0且d＞0

一个无穷等差数列｛a_n｝，公差为d，则｛a_n｝中有有限个负数的充要条件是（）

1. A.
   a₁＞0且d＞0
2. B.
   a₁＞0且d＜0
3. C.
   a₁＜0且d＜0
4. D.
   a₁＜0且d＞0

A.a₁＞0,d＞0                               B.a₁＞0,d＜0

C.a₁＜0,d＞0                               D.a₁＜0,d＜0