Meaning of 'put two and two together'

[kris]

Test No. incompl/advan-71 "Two For You", question 7

I worked out the answer in the end by putting two and two ..........

(a) aside
(b) apart
(c) together
(d) through

Test No. incompl/advan-71 "Two For You", answer 7

I worked out the answer in the end by putting two and two together.

Correct answer: (c) together

Your answer was: incorrect
I worked out the answer in the end by putting two and two apart.
_________________________

I want to know what "...by putting two and two together" means. Thank you!

[Torsten]

If you put two and two together you make a logical connection between two facts.

TOEIC listening, photographs: Among the dunes