What can we say about tariffs? Which of the following claims are true?

a.

Most countries use relatively low levels of average tariffs thanks to the gradual liberalization which took place after the establishment of GATT.

b.

Most tariffs applied by individual countries are of the ad valorem type, i.e. set as a percentage of customs value of the trade commodity.

c.

Tariffs are much easier to compare and eliminate than the non-tariff barriers (NTMS).

d.

All countries use not only import tariffs but also export tariffs and transit tariffs extensively, even now in 2021.