What is the spring force for spring $2$ acting on mass $1($assuming x$1>$x$2)$

Question $1$Select one:

a$.$

$-$k$2*($x$1-$x$2)$

b$.$

k$2*($x$1-$x$2)$

c$.$

x$1*($k$1-$k$2)$

d$.$

$-$x$1*($k$1-$k$2)$