The relation $R(a,b)$ may have duplicate tuples. Which of the following queries has a result that is guaranteed not to have duplicates, regardless of what tuples $R$ contains?

a$.$ SELECT b

FROMR

WHERE $a=b$;

b$.$ SELECT b

FROMR

WHERE b NOT IN $($ SELECT b

FROM R$)$;

c$.$ SELECT a

FROMR

WHERE $a=1$;

d$.$ SELECT $*$

FROMR

WHERE $a>$ b;