What was the primary problem with mortgage-backed CDOs, which triggered the subprime crisis?

Select one:

a.

they were formed by pooling subprime mortgages

b.

mortgage-backed CDOs had nothing to do with the subprime crisis

c.

the mortgages were variable rate loans

d.

the investment bankers underestimated correlation

e.

interest rates increased

f.

they were formed by pooling high quality fixed-rated loans with low interest rates