a. Predefined grid classes like .row and .col-xs-4 are available for quickly making grid layouts. LESS mix ins can also be used for more semantic layouts.
b. Columns create gutters (gaps between column content) via padding. That padding is offset in rows for the first and the last column via negative margin on .rows.
c. Grid columns are created by specifying the number of twelve available columns you wish to span. For example, three equal columns would use three .col-xs-4.
d. All of the above