All of the following are possible sources of error in a scientific investigation except for

1- discovering new variables as the experiment proceeds

2- not controlling all the variables in the experiment

3- adding research resources during an investigation

4- measuring the materials of an experiment incorrectly

The answer to this question should be 3. adding research resources during an investigation 

The answer 1 and 2 are similar. Discovering a new variable mean that you wouldn't control it at the beginning of the research. It would be too late if you try to control it. Incorrect measurement definitely will cause an error.