actress前面用a 还是an?

actress前面用a 还是an? actor和actress前面加a还是an sing writer actress engineer cleaer TV reporter的前面用a或an填空 actress的单数是加a还是加an. 字母A前面用a还是an s前面用a还是用an university前面用“a”还是用“an”? year前面用a 还是用an exciting前面介词用a 还是an mp4前面用an 还是a egg前面用a,还是an? exciting前面用定冠词a还是an engineer 前面用A 还是AN 字母“n”前面用a还是an? ufo前面用a还是an useful前面用an还是a?为什么? 字母u前面用a还是an UFO前面用a an 还是the