Which sentence contains an adverb clause?
a. only 1,600 covered bridges exist in the world.
b. if you visit pennsylvania, take the self-guided, covered-bridge tour.
c. covered bridges remind us of days gone by.
d. the lehigh valley, in eastern pennsylvania, has seven covered bridges.

The sentence that contains an adverb clause is "The Lehigh Valley, in eastern Pennsylvania, has seven covered bridges."

Explanation:

An adverb clause works as a regular adverb and they belong to the same categories, the sentence D has an adverb clause with an adverbial of place, this clauses commonly start with a preposition which in this sentence is in, this tell the place where the things happen or where the things are, then this one specifically describes where the Lehigh Valley is.

correct answers:

c. they use panels.

d. they include images and words.

e. they contain gutters.

your answer is c) experiments with water vapor showed that elements combine in specific proportions.