Quick questions on Nuclear binding energy explained: H2 Physics Modern Physics

The mass of a nucleus is always less than the total mass of its separate protons and neutrons. The difference is the mass defect $\Delta m$:

To compare the stability of different nuclei, divide the binding energy by the number of nucleons:

Plotting binding energy per nucleon against nucleon number $A$ gives a curve that rises steeply for light nuclei, peaks around $A = 56$ (the iron region, the most stable nuclei), then falls gradually for heavy nuclei. A nuclear reaction releases energy when it produces nuclei with a higher binding energy per nucleon, that is, when it moves toward the peak.

Define the mass defect of a nucleus. [2 marks]

A nucleus has a mass defect of $2.0 \times 10^{-28}\ \text{kg}$. Find its binding energy ($c = 3.00 \times 10^8\ \text{m s}^{-1}$). [2 marks]

Explain, using the binding-energy-per-nucleon curve, why energy is released when light nuclei fuse. [3 marks]