What is the domain of the function $y=2 \sqrt{x-6}$ ?
A$-\inftyB$0 \leq x<\infty$
C$3 \leq x<\infty$
D$6 \leq x<\infty$
A$-\infty
B$0 \leq x<\infty$
C$3 \leq x<\infty$
D$6 \leq x<\infty$