The united states canada’s exports.
pays higher prices for
imposes high tariffs on
buys very few of
buys almost all of

Good choice: buys almost all of

Explanation:

The USA is the first trading partner of Canada. It´s the main destination of Canadian exports. In 2018, according to statistics of the USTR, Canada exported good and services to the US for US$ 354.7 billion. To China, its second trade partner, Canada exported US$ 18.4 billion in 2017, according to other sources. It´s easy to see the disproportion.