Helen has been looking for a job over the whole year, feeling she can’t get _________ without work experience.

A．that

B．it

C．one

D．this

Elgamal 公钥体制在原理上基于下面哪个数学基础______。

A．Euler 定理

B．离散对数

C．椭圆曲线

D．歌德巴赫猜想