I put D coz it said crossing over between homologous chromosomes, and I thought they were identical, so if they swap over bits, it wouldn't be any different gentically??????
All the question is asking is which of the four is not a source of variation. Sex linkage actually limits variation. The other three provide variation. B!
I used the cancellation method and came out with B. I think the trick was that people would of seen gametic cells, thought sex-cells and assumed sex-linkage was not the answer