Sequential circuit is a common logic device whose output values, at a given instant, depend upon its input values and the internal state at that instant, and whose internal state depends upon the (66) preceding input values and the preceding internal state. A sequential circuit can assume a finite number of internal states and may therefore be regarded, from an abstract (67) , as a finite (68) .

Combinational circuit is another logic device whose output values, at any given instant, depend upon the input values at that instant. A combinational circuit is a (69) of a sequential circuit whose internal state is not taken (70) .

（70）处填（）。

A．calculation

B．in account

C．audit

D．attention

血红蛋白分子中包含的金属离子是（）

A.镁离子

B.铁离子

C.锌离子

D.锰离子

E.铜离子