Answer:
1.
A. In regions of the world where poverty prevails, starvation and poor hygiene lead to disease.
2.
D. Interesting news is what sells our paper.
Explanation:
Subject-verb agreement occurs when the subject corresponds in number with the form of the verb. In that matter, a singular subject must agree with a singular verb, and a plural subject must agree with a plural verb.
The rest of the sentences are incorrect because their subjects and verbs do not agree in number. Their correct version is the following:
<em>B. Open and honest communication </em><em>contributes </em><em>to a happy relationship.</em>
<em>C. The president or the vice president </em><em>is </em><em>speaking today.</em>
<em>D. The purpose of this essay is to investigate how the design of toothbrushes </em><em>has </em><em>changed over time.</em>
<em>A. A large herd of cows </em><em>was </em><em>grazing by the river.</em>
<em>B. A pound of cookies </em><em>costs </em><em>about a dollar.</em>
<em>C.Each of us </em><em>wants </em><em>to do well in school.</em>