下列外加剂中，不用于提高混凝土抗渗性的是( )。

A．(A) 膨胀剂

B．(B) 减水剂

C．(C) 缓凝剂

D．(D) 引气剂

Even though the main source of ______ exposure for a majority of the human population is the sun, the artificial tanning from sun beds contributes significantly to the total UV risk.

A．extra violent

B．extra violet

C．ultraviolet

D．ultravirus