$1-$Consider the natural join between two relations R$($A$,$ B$)$ and S$($B$,$ C$)$ with the schema and data as shown

below:

A B

a$1$ b$1$

a$2$ b$1$

a$3$ b$2$

a$4$ b$3$

$$R

B C

b$2$ c$1$

b$2$ c$2$

b$3$ c$3$

$$S$$

That is$,$ relation R has two attributes A and B$,$ while relation S has two attributes B and C$.$ Suppose the

natural join query is such that the output schema and result is:$$

A C

a$3$ c$1$

a$3$ c$2$

a$4$ c$3$

If the system uses map reduce to implement this query execution. What would be the key for the reduce

function?

Attribute B

Attribute A

Attribute C

Either the primary key of R or the primary key of S