As a stopping point on the famous Silk Road, Loulan is believed __________ by sandstorms from AD 200 to AD 400.
A：to be gradually covered over
B：to gradually cover over
C：to have been gradually covered over
D：to gradually have covered over
加入WTO十年来，我国遭遇的外贸摩擦也逐年增多，尤其以2010年为最。由于对外贸易依存度高，中国将面临更严峻的国际贸易保护主义的挑战。对此，我们要（ ）
①提高利用世贸组织规则的本领
②完善贸易摩擦预警监控机制
③努力避免经济全球化的影响
④面向国际市场，优化进出口结构
