<span>The elements that are found in a high proportion in Earth's crust are oxygen, aluminium, iron, calcium, potassium, sodium, and magnesium. But for your Option, I think it is B) Oxygen and Silicon. This is because Oxygen makes 46.6% while Silicon makes 27.7% of the Earth crust.</span>
Sister chromosome exchanges are recombination events that occur between homologous chromosomes and are often the result of exposure of the cell to DNA damaging agents.
Cross over, or recombination, is the exchange of chromosome segments between non sister chromatids in meiosis. Crossing over creates new combinations of genes in the gametes that are not found in either parent, contributing to genetic diversity.
It seems you forgot your options, but here are some things found in a chloroplast:
grana
Granal thylakoids
Stroma
Nukleloids (DNA rings)
Ribosomes
Starch granules
membranes
Now, some examples of thing NOT found in a chloroplast:
Endoplasmic Reticulum (that's another organelle)
