Which of the following is not a part of the WTO's stance on trade ?

a . Freer import policy for the cleanest foreign country must be matched by freedom of imports for polluter countries .

b . Any trade barrier that reduces greenhouse - gas emissions is permissible

c. Tariffs that are implemented to conserve natural resources must not be used merely to shut out foreign goods .

d . Any tax on imports must not exceed that on import competing products .