例句與造句

1. Recently they had learned to cast light concrete boats
   最近他們學(xué)會了澆鑄輕型水泥船。
2. Recently they had learned to cast light concrete boats
   最近他們已學(xué)會了澆鑄輕型水泥船。
3. It's difficult to find light concrete in a sentence. 用light concrete造句挺難的