若关系R、S如下图所示，则R与S自然连接后的属性列数和元组个数分别为 (55) ；π1.4(σ3=6(R×S))= (56) 。

（55）处填（）。

A．4和3

B．4和6

C．6和3

D．6和6

---I was trying to carry out the plan, but I failed.

---Well, you _____, for we were uncertain it would do ____.

A．needn’t do that, in fact

B．needn’t have done, in effect

C．needn’t have, in practice

D．needn’t, in time