A manufacturer builds a machine which will address 500 envelopes in 8 minutes. He wishes to build another machine so that when both are operating together they will address 500 envelopes in 2 minutes. The equation used to find how many minutes x it would require the second machine to address 500 envelopes alone, is = ?

• A. $8 - x = 2$$
• B. $\frac{1}{8} + \frac{1}{x} = \frac{1}{2}$$
• C. $\frac{{500}}{8} + \frac{{500}}{x} = 500$$
• D. $\frac{x}{2} + \frac{x}{8} = 1$$