每个健康公民，都应该积极参加义务献血，通常一次献血量为（　　）

A．20～30ml

B．200～400ml

C．1000ml

D．2000～3000ml

Let R and S be the relation respectively:

　　The number of tuples that the result of the expression R co S (Here is the natural join of relations R and S) is

A．3

B．4

C．6

D．9