Question

Which of the following sets is closed under multiplication?
A. {0, 1, 2, 4}
B. {0, 1}
C. {1, 2}
D. None of the sets are closed under multiplication.

Answer 1

0*0=0
0*1=0
1*0=0
1*1=1
so {0,1} is closed under multiplying

but for example {1,2} is not
1*2=2
2*1=2
1*1=1
but 2*2=4 is not in {1,2}