What causes the downward movement along the demand curve of a product (if there is a decrease in it's price compared with it's substitute)?

Both answer A and B have the same narritive but if you select A you are given wrong. Should be looked into.

Assuming that coffee and tea are substitutes, a decrease in the price of coffee, other things being equal, results in a (an)

A. Downward movement along the demand curve for tea.
B. Downward movement along the demand curve for tea.
C. Upward movement along the demand curve for tea.
D. Rightward shift in the demand curve for tea.

With a decrease in the price of a product, people will want to buy more of it ;and buy less of it's substitute.