Gammar question

[gameplayer1234]

Among the [a] earliest telescopes ** were Galilean telescopes, modeled after the simple instruments [c] built by Galileo, the first person [d] having used telescopes to study the stars and planets. No error
Why having used should be to have used. It is said that to clarify but I am just not getting it

[anon67749591]

Quoted from this [Knowsys SAT Blog: Verbs](Knowsys SAT Blog: Verbs):

D is also a verb, but “having used” is a participle, which means it functions as an adjective here. Without a verb in the main clause, this sentence is really a fragment. D is the error, so it is the correct answer.

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