The graph of g(x) is the graph of f(x)=x+6 reflected across the x-axis. Which equation describes function g?

A. g(x)=−6x−6

B. g(x)=−x+6

C. g(x)=x−6

D. g(x)=−x−6

Explain your thinking in getting the answer and I will mark brainliest.