who executes code in .net OPERATING SYSTEM or CLR?

Question

 i have a great confusion about who executes code in .net.
because  i have learnt that code is executed by CLR but a teacher who teaches me .net told
that it is executed by operating system. that's the origin of my confusion.
because according to books it is executed by CLR and according to teacher it is executed by
operating system. i can not make sure who one is true .......
please  explain..