'If you look out the window' vs 'If you look out of the window'

[Saneta]

English Grammar Tests, Elementary Level

ESL/EFL Test #114 "Weather Test (1)", question 7

If you look out the window and it's snowing, it's probably ......... outside.

(a) cool
(b) warm
(c) cold

English Grammar Tests, Elementary Level

ESL/EFL Test #114 "Weather Test (1)", answer 7

If you look out the window and it's snowing, it's probably cold outside.

Correct answer: (c) cold

Your answer was: correct
_________________________

Shouldn't it be:
If you look out of the window...
with added ''of''?

[Mister Micawber]

.
'Of' is optional here.
.
_________________
Native English teacher at Mister Micawber's

[Borislav]

why we can't say : "It is cool outside" ?

[Tomasito]