21.已知n是正整數(shù)，則 的值是（　　　）
A.2n-2                                                      B.2n-1

C.2n                                                         D.2n+1

22.若（a, c）=1，（b, c）=1，則下列結(jié)論不一定正確的是（　　�。�
A.（ab, c）=1                                           B.(a+b, c)=1

C.(ac, a+c)=1                                            D.(c, b+c)=1

23.下列同余式有4個解的是（　　�。�
A.                                       B.

C.                                 D.

24.循環(huán)小數(shù) 化成既約分?jǐn)?shù)為（　　�。�
A.                                                     B.

C.                                                        D.

25.3在40！內(nèi)的最高次冪是（　　�。�
A.18                                                         B.17

C.14                                                         D.10