Answer:
C. Both x is a function of y and y is a function of x
Explanation:
A function is a relationship which assigns to each input (from the domain) value, a unique output (from the range).
y is a function of x means x is from domain, y is from range
x is a function of y means y is from domain, x is from range
1. Given

y is a function of x because for each x there is exactly one y corresponding to this x.
2. Rewrite
expressing x in terms of y:

x is a function of y because for each y there is exactly one x corresponding to this x.