—Have a cigarette, will you ?

—No, thanks. I _________ heavily, but I’ve given it up recently.

A．am used to smoking

B．used to smoking

C．used to smoke

D．has used to smoke

研究疾病与健康状况在人群中分布的决定因素属于()

A．试验性研究 

B．描述性研究 

C．分析性研究 

D．前瞻性研究 

E．队列研究