Why "the others" and not "others"

[Jojo]

English Grammar Tests, Elementary Level

ESL/EFL Test #74 "Other/another/others/the others", question 3

There are four girls in my apartment. Two are called Casey and Amber. ......... are Joanne and Lindsey.

(a) Others
(b) The others
(c) Another
(d) The other

English Grammar Tests, Elementary Level

ESL/EFL Test #74 "Other/another/others/the others", answer 3

There are four girls in my apartment. Two are called Casey and Amber. The others are Joanne and Lindsey.

Correct answer: (b) The others
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Why "the others" not "others"

Jojo

[Mister Micawber]

.
For the same reason we use the in other instances, Learner-- specificity: it is a limited group of four persons, and two have already been named, so there are are only two specific others.
.
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Native English teacher at Mister Micawber's