答案是with
因为
be covered with　　固定用法‘被……覆盖’
加油!不明白再问!be covered by 也有同样的意思啊 怎么不能用呢这个的后面一般接人の……能否分别来几个例句呢 谢谢!be covered with 被…所覆盖,覆盖着,盖满…（强调状态）The grass was covered with frost in the early morning.The tree was covered with beautiful pink blossom.be covered by ... : 由...承担/负担/支付;被...掩护/报道/监视/控制等等,表示抽象的概念, 后面接人, 机构, 或者组织.　　（强调动作）These expenses are covered by the state.