A student investigated how the mass of water in an electric kettle affected the time taken for the water to reach boiling point. The kettle switched off when the water reached boiling point.

Figure 1 shows the kettle.

(a)  The heating element of the kettle was connected to the mains supply.

Explain why the temperature of the heating element increased.

(2)

(b)  Give one variable that the student should have controlled.

(1)

Figure 2 shows how the mass of water in the kettle affected the time taken for the kettle to switch off.

Figure 2

(c)  Suggest why the line on Figure 2 does not go through the origin.

(1)

(d)  Suggest why the results give a non-linear pattern.

(1)

Can someone please answer part (c)? Please ​