1. Rosa sent out more than 100 job application letters. Only three of the companies responded.
The most effective way to combine these sentences would be:
a. Rosa sent out more than 100 job application letters, and only three of the companies responded.
b. Rosa sent out more than 100 job application letters, but only three of the companies responded.
c. Rosa sent out more than 100 job application letters, so three of the companies responded.