SAT writing question

Test Preparation SAT Preparation
1

During the male courtship ritual of the periodical cicada, a type of insect chiefly famous for its long (hibernation, periods of synchronized singing alternate) with periods of silence.

A- hibernation, periods of synchronized singing alternate
B- hibernation, they alternate periods of synchronized singing
C- hibernation that alternate periods of synchronized singing
D- hibernation, periods of synchronized singing alternating
E- hibernation alternate periods of synchronized singing

I know the answer is A but I got confused by “periodical cicada”. I didn’t think an insect could be “periodical” so I thought periodical was describing a behavior of a “a type of insect”.
I would understand the phrase “cicada’s periodical singing and hibernation” but not the phrase " a periodical cicada"

periodical definition: occurring or appearing at intervals; occasional.

Point being, I got confused by the adjective periodical and the subsequent appositive following cicada-- “a a type of insect chiefly famous for its long hibernation”

While some scholars ‘‘claim’’’ that the settlement was established ''around ‘’ the year 1250 , (other) doubt (there to be ) human activity in the area before 1400.

What’s the difference between there was and there to be?

2

That there exist an infinite number of worlds outside our solar system were first hypothesized by the thriteenth-century philosopher Giordano Bruno.

I know that were should’ve been was but should exist be exists as well? What is exist referring to?

3

Some cicadas only emerge from hibernation periodically (every 7 years or so), but some cicadas come out every year.

“There to be” is present time reference, which can’t be applied to events before 1400. For past time events with specified time references, you need simple past.

4

“Exist” is the verb for the plural subject “an infinite number of worlds.” It’s s-v inversion. Oh, and “a/n number of” is plural, whereas “the number of” is singular.