Which sets include only like terms? choose all answers that are correct.
a. {12c, -25c, 1.876c, 2/9c
b. {26def, -4def, 2.92def,12/19def}
c. {-1.24rs, 17rs, 9rs, 2 4/9 rs}
d. {-11xyz, -3.07xy, -42yz, -6/17xz }