What does not only but also mean?

[Liam]

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #85 "Verbal Skills Test", question 3

Not only did he pay for them to go on holiday but he ......... provided them with spending money.

(a) addition
(b) also
(c) either
(d) too

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #85 "Verbal Skills Test", answer 3

Not only did he pay for them to go on holiday but he also provided them with spending money.

Correct answer: (b) also
_________________________

Dear Sir/Ma'am,
Please get me provide the explanation of this question.

thank you.

[Mister Micawber]

The phrase being tested is 'not only...but also'.
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Native English teacher at Mister Micawber's