【单选题】一般认为脊髓活动的_________机制可引起牵涉性疼痛。
【单选题】For how long can a reader keep the book before he renews it?
【单选题】______ When the reader saw the advertisement he was sure he could get it and applied for the job with a false name.
【单选题】We ________ here two hours ago and we ________ here for two hours.
A.
have arrived; have been
【单选题】For how long can a reader keep the book before he renews it?
【单选题】增塑剂的主要作用机理是降低聚合物分子间的相互作用力,增加聚合物分子链的移动性,下面哪种机理是极性增塑剂的主要增塑机理?