How does SELECT from two tables separated by a comma work? (SELECT * FROM T1, T2)

Given 2 tables T1 and T2.

T1   T2 
---------
A    1 
B    2
C    3

You make a query:

SELECT * 
  FROM T1, T2

What is the no: of rows that are fetched from this query?

(a) 4
(b) 5
(c) 6
(d) 9

Answer is : 9

Question:

Why is the answer "9"?