**17) Which of the following sets in infinite?**

(A) the set of student of your class

(B) the set of all schools in the USA

(C) the set of natural numbers between 3 and 10

(D) the set of rational numbers between 3 and 10

**18) i**

^{-3}= ___(A) i

(B) 1

(C) -1

(D) -i

** 19) If ^{n}P_{2} = 30 then n = ?**

(A) 32

(B) 6

(C) 5

(D) 28

** 20) If ^{n}C_{8} = ^{n}C_{12} then n = ?**

(A) 20

(B) 4

(C) 96

(D) none of these