After the following code sequence is executed, what are the contents at ArrayList index 1 in a?

After the following code sequence is executed, what are the contents at ArrayList index 1 in a?

ArrayList<Integer> a = new ArrayList<Integer>();

a.add(7);

a.add(10);

a.add(21);

a.set(1, 4);

a.remove(0);

The choices are :

a.10

a.7

c.4

d.21