What Does the Phrase, "Terms of Trade" Refer To

What does the phrase, "terms of trade" refer to?

A) it refers to the agreed upon terms in a contract of sale.
B) it refers to the rate of exchange between an export good and an import good.
C) it refers to the level of tariffs imposed on an imported good.
D) it refers to the length of time between the sale and the actual delivery date.