A $0\mathrm{.}75-kg$ block slides on a rough horizontal tabletop. Just before it hits a horizontal ideal spring its speed is $3\mathrm{.}5\frac{m}{s}.$ It compresses the spring $5\mathrm{.}7$ cm before coming to rest. If the spring constant is $1200\frac{N}{m},$ the thermal energy of the block and the tabletop must have:

a$)$ not changed

b$)$ decreased by $1\mathrm{.}9$ J

$\frac{1}{2}k{x}^2$

$1496\mathrm{.}25$

c$)$ decreased by $2\mathrm{.}6$ J