As More Capital Per Worker Is Added, a Per-Worker Production

As more capital per worker is added, a per-worker production function becomes:

A) flatter because capital is subject to diminishing marginal returns
B) flatter because labor is subject to diminishing marginal returns.
C) flatter because capital is subject to increasing marginal returns.
D) steeper because capital is subject to diminishing marginal returns.
E) vertical as output per worker does not change with change in capital per worker.