As countries ______ to increase yields on existing croplands through intensified use of water, energy, and fertilizers, the cost of commodities will rise.

A．encounter

B．enchant

C．enlarge

D．endeavor

病人平卧抬高下肢，排空静脉血，在大腿根部扎止血带阻断大隐静脉，然后让病人站立，10秒内放开止血带，若出现自上而下的静脉逆向充盈，提示

A．交通静脉瓣膜功能异常

B．下肢深静脉通畅

C．小隐静脉瓣膜功能不全

D．大隐静脉瓣膜功能不全

E．下肢浅静脉通畅