Are you going to the concert ______ on Friday evening in the people’s Square?

A．to hold

B．to be holding

C．to have held

D．to be held

重量法测定水中全盐量时，水中全盐量按下式计算：

C（mS／L）=（W—W0）×106／V

试解释式中W、W。和V的含义及单位。