CIRCLE ONE:

interaction?

$>>>s1=\{1,2\}$

$>>>s2=s1$

a$.\{1,2\}\{1,2\}$

$>>>s1.$add $(3)$

$>>>s2.$remove $(3)$

b$.\{1,2\},\{1,2,3\}$

$>>>$ print $(s1,s2)|$

C$.\{1,2,3\}\{1,2\}$

d$.\{1,2,3\}\{1,2,3\}$