请问如何理解这题的解析？

问题如下：

A colleague of the bank analyst suggested adding the returns on the Dow Jones Industrial Average (DJIA) index as an additional explanatory variable. The new regression results show that the R² increased and the adjusted R² decreased. What conclusion can be drawn from these results?

选项：

The increased R2 gives an inflated estimate of how well the regression fits the data

The decreased adjusted R² suggests that the coefficient of the returns on the DJIA is not significant

The increased R² indicates an improvement in the regression

The decreased adjusted R² suggests the existence of omitted variable bias in this regression

解释：