21. What is a clause?

A clause is a part of the sentence that contains a verb. A typical clause consists of a subject and a predicate, the latter typically a verb phrase, a verb with any objects and other modifiers.A simple sentence usually consists of a single finite clause with a finite verb that is independent.

halibut, flounder, tuna, and trout are all fish. so your first answer is 1.) fish. and since fish is edible, your next answer would be 3). edible

to describe those fishes more thoroughly, some of those fishes live in fresh water, and some of the fishes live in salt water, and that is not a thing that they all have in common. the question is asking what is common with all of the fish, so answer choices 2). fresh water and 4). salt water are not right.