If something happens to alter the quantity demanded at any given price, then

A) the demand curve becomes steeper.
B) the demand curve becomes flatter.
C) the demand curve shifts.
D) we move along the demand curve.

When the quantity demanded changes at any given price, it indicates a change in demand, not just a change in the quantity demanded. This is represented by a shift in the demand curve, either to the left or to the right, depending on whether demand decreases or increases, respectively. Changes in factors like consumer preferences, income, prices of related goods, or expectations about future prices can cause such shifts. Moving along the demand curve would imply a change in the quantity demanded due to a change in price, not a change in demand itself.