(61)There are several periods in a computer，the shortest period is

A)Instruction period B)Machine period C)Beat period D)CPU period

(62)Which set is empty？

A)｛x|x is a real number and x2=9

B)｛x|x is a real number and x2——1=0｝

C)｛x|x is a real number and x21=0｝

D)｛x|x is a real number and x=2x1｝

(63)What is the relation represented in the exhibit shown below？

A)R=｛(1，2)，(1，3)，(1，4)，(1，5)｝

B)R=｛(1，1)，(2，2)，(3，3)，(4，4)，(5，5)｝

C)R=｛(1，2)，(1，3)，(1，4)，(2，3)，(4，1)，(4，5)，(5，5)｝

D)R=｛(1，2)，(1，3)，(1，4)，(2，2)，(2，3)，(4，1)，(4，4)，(4，5)｝

(64)What is the contrapositive of the following implication？
“If it is raining，then I get wet.”

A)If I get wet，then it is raining.

B)If I am wet ，then if is raining.

C)If it is not raining,I do not get wet.

D)If I do not get wet，then it is not raining.

(65)Which property does R posses？