(i) Calculate the resistance $R_1$ of the filament of lamp $L_1$ using the formula $R_1=\frac{V_1}{I_1}$. Include the correct unit in your answer.

In the second part, lamp $L_2$ is connected in series with lamp $L_1$. The potential difference $V_2$ is measured across $L_2$, and the new current $I_2$ in the circuit is measured:

Calculate the resistance $R_2$ of the filament of lamp $L_2$ using the formula $R_2=\frac{V_2}{I_2}$.

In the third part, lamp $L_3$ is connected in series with lamps $L_1$ and $L_2$. The potential difference $V_3$ across $L_3$ and the circuit current $I_3$ are measured:

Calculate the resistance $R_3$ of the filament of lamp $L_3$ using the formula $R_3=\frac{V_3}{I_3}$.

Calculate the sum of the three resistances, $R_1+R_2+R_3$. Express your answer to a suitable number of significant figures (2 or 3 significant figures).

Some students make the following suggestions about the results of the experiment:

• Suggestion A: $R_1+R_2+R_3$ is equal to $3\times R_1$.
• Suggestion B: $R_1+R_2+R_3$ is less than $3\times R_1$.
• Suggestion C: $R_1+R_2+R_3$ is greater than $3\times R_1$.

State which suggestion, A, B, or C, agrees with the calculated resistances. Justify your answer using your numerical values.

State a variable that the student could change to determine whether this relationship always holds for these three lamps, and describe how this variable can be changed.