"Auto industry" was the sector of the economy in the 1920s in which falling demand and raising debt were a problem. The auto industry was the industry that was having huge problem in regards to selling of new cars. I hope that this is the answer that has actually come to your desired help.
I believe the answer is: Consumer goods (such as agriculture) and property
At 1920, our society just learnt to industrialized the production process using the development of technology. This create a massive oversupply for consumer goods that lead to falling demand because most consumers have more than they could normally consume.
