If the variables for the CSP is defined as:

X$=\{$a$1,$a$3,$d$1,$d$2\}$

what does the content of the set represent?

Select one or more:

a$1$

corresponds to $1-$across

a$3$

$$ corresponds to $3-$across

d$1$

$$ corresponds to $1-$down

d$2$

corresponds to $2-$down

a$1$

corresponds to $1-$across

a$2$

$$ corresponds to $2-$across

d$1$

$$ corresponds to $1-$down

d$2$

corresponds to $2-$down

a$1$

and a$3$

are placeholders for the first and third letters used across all words

d$1$

and d$3$

are the domain sizes for the words placed vertically

a$1$

is the first letter of every word used in the puzzle

a$3$

is the last letter of every word used in the puzzle

d$1$

refers to the number of words that can fit horizontally

d$2$

refers to the number of words that can fit vertically

a$1$

and a$3$

as actions for determining crossword intersections

d$1$

and d$2$

as difficulty levels for the words placed in the puzzle