Final answer:
The first "killer app" that justified the cost of a computer all by itself was the spreadsheet software.
Step-by-step explanation:
The first "killer app" that justified the cost of a computer all by itself was the spreadsheet software. Spreadsheet software allows users to organize, analyze, and manipulate data in a tabular format.
It was a groundbreaking development that revolutionized the way businesses handled their financial data and calculations. The most well-known example of spreadsheet software is Microsoft Excel, which is still widely used today.