The volunteers are doing _____ difficult and important work; they share ____ common goal to help people in need. A. a; a B. a; the C. 不填; the D. 不填; a 答案:D在数列中,.现就数列给出下列命题: ①不存在实数,使得均为正数;②存在实数,使得均为负数; ③不存在实数,使得是等比数列; ④存在实数,使得是等差数列. 其中真命题的个数是( ) A.1 B.2 C.3 D.4