Suppose there are 6 memory blocks $B_1$, $B_2$, $B_3$, $B_4$, $B_5$, and $B_6$ with corresponding sizes of $300\,\mathrm{KB}$, $600\,\mathrm{KB}$, $380\,\mathrm{KB}$, $200\,\mathrm{KB}$, $750\,\mathrm{KB}$, and $125\,\mathrm{KB}$. There are 5 processes $P_1$, $P_2$, $P_3$, $P_4$, and $P_5$, with corresponding request sizes of $115\,\mathrm{KB}$, $500\,\mathrm{KB}$, $358\,\mathrm{KB}$, $200\,\mathrm{KB}$, and $375\,\mathrm{KB}$. When applying the best-fit algorithm, which memory block is process $P_5$ assigned to?