GMAT Sentence Correction (SC) | Re: Though being tiny, blind, and translucent, a recently discovered

Uses of pronoun "that":

1. as a demonstrative pronoun:
1a. as adjectives:That pen is mine.
1b. as copy: The car you gifted me is older thanthat standing outside (creates a copy ofcar ).

2. as a relative pronoun:
I wanted to buy the housethat was destroyed by earthquake yesterday.
I do not like dogsthat bark.

The usage 1a and 1b can only be singular - for plural "those" is required:
That pens are mine... wrong
The cars you gifted me is older thanthat standing outside
...

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