The first textbooks _____for teaching English as a foreign language came out in the 16th century. A. having written B. to be written C. being written D. written
BEIJING (AP) — Sandstorms whipping across China shrouded(遮蔽) cities in an unhealthy cloud of sand Monday, with winds carrying the pollution outside the mainland as far as Hong Kong and Taiwan.
的定义域为______________________________。 
解析： 
