Amutation is made once in every _ nucleotides copied

Mutation is made once in every thousand nucleotides copied. 

Mutation creates different versions which have the same genes which are known as alleles.
They create variation in protein-coding. We put sunscreen to protect us from a mutation in somatic cells.

Answer is controlled experiment.

An experiment in which all the variables are same or constant except for one variable is called controlled experiment. In this type of experiment a set of data is considered for a control group and one or more other groups are examined. All conditions are identical to the control group and each other except one variable.

Example of a controlled experiment:

How the type of soil affects the time of seed germination?

To address this question, a controlled experiment is set up.  For the experiment four identical pots were taken and filled each with a different type of soil.  Bean seeds were planted in each pot, place the pots in a sunny window, water them, and measured how long it takes for the seeds in each pot to germinate.

This is a controlled experiment because the aim was to keep every variable constant except the type of soil used.