What is the controlled variable for: 1.Does the amount of leaves on a tree affect how many birds build their nests in it? 2.Does the acid level of the lake affect how many fish live there?

Case I - Number of leaves

Case II - Acid level of lake

Explanation:

A controlled variable is the one which is kept constant or unchanged  throughout an experiment.  

In the question –  

For case I –  

Here the impact of number of leaves on a given tree is studied on the number of nest build by birds.  

Thus, the number of leaves is a constant variable that shall remain constant throughout and hence it is a controlled variable.  

For case II –  

Just like in the above case, in this case too the acid level of the lake shall remain constant as the effect of acid level of lake is studied on the survival instinct of fishes in the lake.  

the answer to this question is 3.