Interpretation of Multiple Regression Results

As variables are added to a multiple regression, it is expected that:

Correct! As the model becomes more crowded with variables each working to explain the dependent variable, there is less variation being attributed to each. This causes existing independent variables to become less significant.

Incorrect. The inclusion or exclusion of coefficients is totally unrelated to the variation observed in the dependent variable.

Incorrect. Adding more regressors cannot reduce the amount of variation, which is explained by the regressors in total. Even randomness will cause a regressor to add some explanation to the model.

existing regressors become less significant.

variation in the dependent variable will decrease.

less "explanation" is offered of the dependent variable.