If a problem states "$y$ is directly proportional to $x$," what can you immediately infer about $y$ when $x=0$?

• A. $y$ can be any constant because direct proportionality only applies to nonzero $x$-values unless specified otherwise
• B. $y$ will equal $k$ because the constant of proportionality determines the value when $x=0$
• C. $y$ must be undefined because direct proportionality makes $y$ dependent on the reciprocal of $x$ whenever $x$ is zero
• D. $y$ must equal $0$ because $y=kx$ and $k$ is finite, so $y=0$

Answer: (D)