Given the following piece of code:
public class School{
        public abstract double numberOfStudent();
which of the following statements is true?

Which of the following class definitions defines a legal abstract class?

Which of the following declares an abstract method in an abstract Java class?

Which of the following statements regarding abstract classes are true?

Suppose A is an abstract class, B is a concrete subclass of A, and both A and B have a default constructor. Which of the following is correct?
1. A a = new A();
2. A a = new B();
3. B b = new A();
4. B b = new B();