ABC Bank has a capital of $$50$ and a leverage ratio of $3.$ It earns $5\%$ on its assets but pays $4\%$ on its liabilities. What is ABC Bank's profit per unit of capital invested? $($round to the nearest integer in $\%)$

a$.$

$7\%$

b$.$

$4\%$

c$.$

All of the answers here are incorrect

d$.$

$1\%$

e$.$

$8\%$