There are three classes $X, Y$ and $Z$. The average age of the students of classes $X$ and $Y$ together is $20$ years. The average age of the student of classes $Y$ and $Z$ together is $35$ years. The average age of the students of classes $X$ and $Z$ together is $15$ years. The average of all the three classes can be:

In the following question statement followed by two courses of action numbered I and II is given. A course of action is a step or administrative decision to be taken for improvement, follow-up or further action in regard to the problem, policy etc. On the basis of the information given in the statement, you have to assume everything in the statement to be true, then decide which of the suggested courses of action logically follow(s) for pursuing. 

Statement: 
There is an alarming increase in the number of people suffering from malaria in many parts of the city. 

Causes of action: 
I. The municipal corporation has advised all the government hospitals to store an adequate supply of Malaria drugs. 
II. The municipal corporation has asked people to use mosquito repellents and keep their premises clean.