Which of the Following Is an Error Made by Commercial

Which of the following is an error made by commercial banks in 1920s that ​caused depositors to lose money and forced regulators to impose restrictions?

A) ​Banks sold securities in the primary market.
B) ​Smaller banks merged to form larger banks.
C) ​Banks issued loans to a number of firms that went bankrupt during the Great depression.
D) ​Banks did not diversify their activities and were engaged only in banking activities.