Question: What is a safe operation on a `std::cell:UnsafeCell`?

1. A `&mut T` reference is allowed. However it may not cpexists with any other references. and may be created only in single-threaded code.
2. `UnsafeCell` provides thread-safety. Therefore, creating `&T` references from multiple threads is safe.
3. The only safe operation is the `.get()` method, which returns only a raw pointer.
4. Non. `UnsafeCell` only allows code that would otherwise need unsafe blocks to be written in safe code.

Answer: The correct answer of the above question is Option C:The only safe operation is the `.get()` method, which returns only a raw pointer.