You are developing a web page that will be divided into three vertical sections. The main content of the site will
be placed in the center section. The two outer sections will contain advertisements.
You have the following requirements:The main content section must be set to two times the width of the advertising sections.
The layout must be specified by using the CSS3 flexible box model.
You need to ensure that the visual layout of the page meets the requirements.
Which CSS3 property should you use?

A.
box-orient

B.
box-flex-group

C.
box-flex

D.
box-direction

Explanation:
box-flex
Values: 0 | Any integer
The flexibility ratio for this child. If a child had 1 and its sibling had 2, any additional space in the parent box
would be consumed twice as much by the sibling. It defaults to 0 which is inflexible.
Quick hits with the Flexible Box Model
http://www.html5rocks.com/en/tutorials/flexbox/quick/