The majority of doctors _____ smoking is harmful to health. [ ]

a. are believed

b. had believed

c. has believed

d. believe

高速公路、一级公路和城市快速路、主干路所用石料表层抗滑性能至关重要。根据“六五”及“七五”国家攻关课题研究成果，其中（）是必须进行测试的指标。

A.冲击值

B.压碎值

C.磨光值

D.吸水率