[单项选择]The main purpose of these researchers is ______
A. to help to destroy old cells and boost new ones B. to alter the natural process known as apoptosis C. to find out ways to protect aging brain cells D. to compare different levels of proteins
参考答案:D 答案解析:根据提问，先找到关于研究目的的段落：That prompted researchers to look at whether cutting calories might also play a ro
第19题: [单项选择]关于镇痛作用强弱排列顺序正确是（） A. 美沙酮最强，哌替啶次之，吗啡最弱 B. 美沙酮最强，吗啡次之，哌替啶最弱 C. 吗啡最强，哌替啶次之，美沙酮最弱 D. 吗啡最强，美沙酮次之，哌替啶最弱 E. 哌替啶最强，吗啡次之，美沙酮最弱 参考答案:B 答案解析:盐酸哌替啶镇痛作用为吗啡的1/10，盐酸美沙酮镇痛作用为吗啡的2～3倍。
