由于网络存在多个CA中心，因此提出了（）服务。
证书链
4元线性方程组的基础解系所含解向量的个数为().
Choose the closest paraphrased version after the following sentence or the italicized part. Blacken the letter corresponding to your choice on the ANSWER SHEET.
I stared at the canteen as if it were a mirage.（ ）
