Identify the expressions with nonnegative limit values.
a. lim x-->3 x^2-10x+21/x^2
b. lim x-->3/2 2x^2-5x-12/2x+3
c. lim x-->2 x-2/x^2-2
d. lim x-->11 x^2+6x-187/x^2+3x-154
e. lim x-->3 x^2-8x+15/x-3
f. lim x-->5/2 2x^2+x-15/2x-5