Scientists believe that, unlike most other species of barnacle, turtle barnacles (Chelonibia testudinari) can dissolve the
cement-like secretions they use to attach ______ to a sea turtle shell, enabling the barnacles to move short distances across
the shell’s surface.
it
themselves
them
itself
Choice B is the best answer. The convention being tested is pronoun-antecedent agreement. The plural reﬂexive pronoun
“themselves” agrees in number with the plural antecedent “turtle barnacles,” correctly indicating what is attached to a sea
turtle shell.  
Choice A is incorrect because the singular pronoun “it” doesn’t agree in number with the plural antecedent “turtle barnacles.”
Choice C is incorrect because it results in an unclear and confusing sentence. In this context, it’s unclear what the plural
pronoun “them” refers to. Choice D is incorrect because the singular reﬂexive pronoun “itself” doesn’t agree in number with
the plural antecedent “turtle barnacles.”