Indicate the common factor.

a) [[$ 5x $]] and [[$ 3x $]]

Answer: The common factor of the monomials is .

b) [[$ 2a $]] and [[$ 4b $]]

Answer: The common factor of the monomials is .

c) [[$ 7y $]] and [[$ -8y $]]

Answer: The common factor of the monomials is .

d) [[$ 3 $]] and [[$ 18c $]]

Answer: The common factor of the monomials is .