What two systemic issues in the mortgage and financial sectors contributed to the $2008$ MBS crisis? $($You will get $50\%$ of the mark for each correct answer, and $-33\mathrm{.}33\%$ of the mark for each incorrect answer$).$

a$.$ Excessive regulatory oversight in mortgage lending.

b$.$ Misalignment of risk and return in mortgage$-$backed securities$.$

c$.$ Illusion of safety in investment due to external ratings and insurances.

d$.$ Stable global real estate markets ensuring investment security.

e$.$ Conservative investment strategies by large financial institutions.