Your income response for bicycle riding changes with the amount of income you earn. At low levels of income, you view bicycle riding as an inferior good and substitute other types of transportation as your income rises. However, you view bicycle riding as a normal good after your income rises above a particular level. What shape does your Engel curve for bicycle riding have?

a) vertical line

b) horizontal line

c) C-shaped

d) upward sloping

e) none of the above.

The correct answer is A). Could you explain why?