The $2007$ financial crisis was caused, in part, by:

a$.$

the inaccurate ratings of collateralized debt obligations.

b$.$

the continued enforcement of the provisions of the Glass$-$Steagall Act.

c$.$

the excessive reserves maintained by the issuers of credit default swaps.

d$.$

the refusal of banks to make sub$-$prime mortgages in the years prior to $2007.$