20 pts and brainliest... which best identifies the adverb clause? (Key, the capital letters mean its underlined since i couldnt underline the words.)

a. IF YOU VISIT PENNSYLVANIA, take the self-guided, covered-bride tour.

b. only 1,600 covered bridges EXIST IN THE WORLD.

c. covered BRIDGES REMIND US of days gone by.

d. the leigh valley, IN EASTERN PENNSYLVANIA, has seven covered bridges.