17.
Since the are using the same battery on the experiment and the only think we are changing in the number of coils wrapped around the electromagnet the independent variable is this. Therefore, the answer is b.
18.
The dependent variable of the experiment is number of paperclips the magnet picks up. Therefore, the answer is c.
20.
According to the graph when there are 75 coils wrapped around the magnet picks up 10 paperclips, therefore, the answer is c
