This question is about chemical cells.
A student investigated the voltage produced by different chemical cells using the apparatus shown below.

The student used the following method:
Table 1 shows the student's results:
| Electrode Y | Voltage of the cell (V) |
|---|---|
| zinc | 1.56 |
| iron | 1.24 |
| copper | 0.46 |
Suggest two variables the student should keep the same to make the investigation valid.
Write the three metals used for Electrode Y in order of reactivity from most reactive to least reactive, using Table 1.
Predict the voltage if Silver is used as Electrode Y, and give one reason for your answer.
Describe how to make a 9 V9\text{ V}9 V battery using 1.5 V1.5\text{ V}1.5 V cells.