Given the following system of equations:

3x + 5y = 30
x + 3y = 10

which action creates an equivalent system that will eliminate one variable when they are combined?

a. multiply the second equation by −1 to get −x − 3y = −10.
b. multiply the second equation by −3 to get −3x − 9y = −30.
c. multiply the first equation by −1 to get −3x − 5y = −30.
d. multiply the first equation by −3 to get −9x − 15y = −90.