以下都是C++语言标识符的是（ ）。

A． 若在重定义虚函数时使用了virtual，则该重定义函数还是虚函数

B． 虚函数不能声明为友元

C． 子类必须重定义父类的虚函数

D． 虚函数不能是static的

Drug use is rising dramatically among the nation’s youth after a decade of decline. From 1993 to 1994, marijuana use among young people (1) from 12 to 17 jumped 50 percent. One in five high school seniors (2) marijuana daily. Monitoring the Future, which (3) student drug use annually, reports that negative attitudes about drugs have declined for the fourth year in a row. (4) young people see great risk in using drugs. Mood-altering pharmaceutical drugs are (5) new popularity among young people. Ritalin, (6) as a diet pill in the 1970s and now used to (7) hyperactive children, has become a (8) drug on college campuses. A central nervous system (9) , Ritalin can cause strokes, hypertension, and seizures. Rohypnol, produced in Europe as a (10) tranquilizer, lowers inhibitions and suppresses short-term memory, which has led to some women being raped by men they are going out with. (11) taken with alcohol, its effects are greatly (12) . Rock singer Kurt Cobain collapsed from an (13) of Rohypnol and champagne a month before he committed (14) in 1994. In Florida and Texas, Rohypnol has become widely abused among teens, who see the drug as a less expensive (15) for marijuana and LSD. Alcohol and tobacco use is increasing among teenagers, (16) younger adolescents. Each year, more than one million teens become regular smokers, (17) they cannot legally purchase tobacco. By 12th grade, one in three students smokes. In 1995, one in five 14-year-olds reported smoking regularly, a 33 percent jump (18) 1991. Drinking among 14-year-olds climbed 50 percent from 1992 to 1994, and all teens reported substantial increases in (19) drinking. In 1995, one in five 10th graders reported having been drunk in the past 30 days. Two-thirds of high school seniors say they know a (20) with a drinking problem.

3（）

A.studies

B.researches

C.surveys

D.examines