Below each of the following incomplete sentences are two expressions.
complete each sentence by choosing the expression that is parallel in form with an expression
that is already a part of the sentence.

moderate control of tsetse flies can be achieved by sterilizing them with radiation or
a. by spraying them with insecticide
b. spray them with insecticidal sprays
tsetse flies feed by piercing the skin with their mouthparts and .to suck the blood of animals and humansb. sucking the blood of animals and humans

1. by spraying them with insecticide

2. sucking the blood of animals and humans

Explanation:

In the first statement, it is said that the tsetse flies can be controlled by sterilizing. The statement form is similar to the first option where the verb of the sentence is is in the same tense and the subject of the sentence comes after the present continuous verb form.  

Likewise, in the second sentence the verb tense coincides with the second sentence i.e option B

Hence the correct answers are

1. by spraying them with insecticide

2. sucking the blood of animals and humans