云杉林被采伐后的恢复演替可延续达几十年，有时达几百年，这种恢复演替叫做______；
参考答案：长期演替
The companies are working together to create _________ they hope will be the best means of transport in the 21st century.
A．which
B．that
C．what
D．who
无嗜睡作用的H1受体拮抗剂是
A．马来酸氯苯那敏B．西替利嗪C．赛庚啶D．富马酸酮替芬E．雷尼替丁
