添加Firebase-Core时出现appcompat错误

Question

当我将一项FiberBase服务添加到gradle时，appcompat会出现以下错误

Error: All com.android.support libraries must use the exact same version
specification (mixing versions can lead to runtime crashes). 
Found versions 28.0.0-alpha1, 26.1.0. Examples include
`com.android.support:animated-vector-drawable:28.0.0-alpha1` and
`com.android.support:support-media-compat:26.1.0` 

请引导我。