I attach a metal block to a vertical spring and it stretches by $6$ cm$.$ If I replace the block by another one twice the mass, the spring will stretch

a$)$ another $3$ cm$,$ making the total stretch $9$ cm$.$ b$)$ another $6$ cm$,$ making the total stretch $12$ cm$.$ c$)$ another $12$ cm$,$ making the total stretch $18$ cm$.$ d$)$ more than another $12$ cm$.$