Which of the following are sets are subrings of the field R of real numbers?

a) A={m+n(2)^(1/2)|m,n in the integers and n is even}

b)B={m+n(2)^(1/2)|m,n in the integers and m is odd}

c)C={a+b(2)^(1/3)|a,b in the rationals}

d) D={a+b(3)^(1/3)+c(9)^(1/3)|a,b, c in the rationals}

My problem is getting started. I know a set R is a subring if it is closed under addition and multiplication, if a is in R, the -a is in R, R contains the identity