Help wat does ctrl+u means

Convert the quartstogallons class to an interactive application. instead of assigning a value to the number of quarts, accept the value from the user as input.

Match the following errors with their definitions. a. #name b. #value c. #ref d. 1. when a formula produces output that is too lengthy to fit in the spreadsheet cell 2. when you enter an invalid cell reference in a formula 3. when you type text in cells that accept numeric data 4. when you type in a cell reference that doesn’t exist

What are "open-loop" and "closed-loop" systems