请教03年10月10题

为什么 我们不能说 a

(10)Helium is not flammable and ,next to hydrogen ,is......

(a)known the lightest gas

(b)lightest the known gas

(c)the lightest gas known

(d)the known gas lightest

为什么么不能说Helium is known the lightest gas ？ 难道 要加as ？？？

一般的搭配是be known as