In an Offer Curve Graph with Country A's Exports on the Horizontal

Multiple Choice

In an offer curve graph with country A's exports on the horizontal axis and country B's exports on the vertical axis, which one of the following events will shift or pivot country B's offer curve downward (or to the right) ?

A) decreased demand by A for B-goods
B) technological improvement in B's industries
C) imposition of a tariff by A
D) imposition of a tariff by B