7. Which one of the following is true about age-hardenable 2000-series aluminium alloys? A. The increase in strength associated with the ageing heat-treatment is due to diffusion of the alloy element atoms to the grain boundaries. B. The increase in strength associated with the ageing heat-treatment is due to the alloy element atoms being uniformly distributed on the lattice within the grains. C. During the ageing heat-treatment the alloy strength decreases slightly. D. During the ageing heat-treatment the alloy partially melts. E. For these alloys the main alloy element added to aluminium is Cu,,