Which two demonstrate the valid usage of the keyword synchronized?

Which two demonstrate the valid usage of the keyword synchronized?

A.
interface ThreadSafe {
synchronized void doIt();
}

B.
abstract class ThreadSafe {
synchronized abstract void doIt();
}

C.
class ThreadSafe {
synchronized static void soIt () {}
}

D.
enum ThreadSafe {
ONE, TWO, Three;
synchronized final void doIt () {}
}

Explanation:

The Java programming language provides two basic synchronization idioms:
synchronized methods and synchronized statements.
To make a method synchronized, simply add the synchronized keyword to its declaration.