Which of the following sentences are statements?

a) 1+1 = 2
b) the number 16 is an odd number
c) an even number can be written in the form 2n, where n is a natural number
d) the solution of the equation 3x=18 is a natural number
e) ( a-b)²  ≥ 0 for every rational a and b
f) ab > 0 if a and b are of the same sign
g) a² > 0 if a is a negative number
h) - a² < 0 if a is a negative number