Set off and set on?

[Andrea]

Test No. incompl/inter-85 "Simple Present (2)", question 8

It was late April and the sun had disappeared behind dark heavy rain clouds but Andrew had already set ......... for the local pub.

(a) up
(b) off
(c) to
(d) by

Test No. incompl/inter-85 "Simple Present (2)", answer 8

It was late April and the sun had disappeared behind dark heavy rain clouds but Andrew had already set off for the local pub.

Correct answer: (b) off

Your answer was: incorrect
It was late April and the sun had disappeared behind dark heavy rain clouds but Andrew had already set by for the local pub.
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i would like to know the meaning of set of, set on, and set by

thanks

[Alan]

Set off is start a journey
Set on is attack
Set by is not really a so-called phrasal verb as the other two are and would be used as a simple passive: The questions were set BY the examiner
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