MSCORELIB.dll
System.Data is a namespace, System.Data.DLL (the file) is an assembly.
mscorlib.dll
If you would like to see then find below the path details:-C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework.NETFramework\v4.7.2
Note - .NETFramework could be different as per your IDE version. Thanks
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System.Data.Dll
namespace