Step 1: Think about the objective
Both coefficients are positive, so z is smallest where both x and y are as small as the constraints allow.
Step 2: Lowest corner
The lowest allowed $y$ is $60$ and the lowest $x$ is $0$, giving $(0,60)$ and $z = 300$. Every other corner gives a larger value. Option (C).
Final Answer:
(0, 60).
\[ \boxed{\text{(C)}\ (0,60)} \]