Each of the questions given below consists of a question followed by three statements. You have to study the question and the statements and decide which of the statement(s) is/are necessary to answer the question? 
What is the speed of the train? 
I. The train crosses a signal pole in 18 seconds. 
II. The train crosses a platform of equal length in 36 seconds. 
III. Length of the train is 330 metres.