To understand this question, you first need to determine the meaning of each value. The 320 is the total charge, which includes the money paid per hour(x) and the 70 dollars paid for the parts. The 70 dollars acts as a constant since there is no variable attached to it that will change its value. X is what is paid each hour and will be paired up with five since five hours of work is done.
Now time to write your equation. The 320 includes your 70. You don't want that since you want to find just the value of the five hours of work. So, the correct choice is B.