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menstruation happens when the line breaks down... the graph says that the thickness of the lining is decreasins iow it is breaking down.

the question says that the death rate and birth rate should be equak 'throughout' those years...what you are saying is that the population is equal in the first and last year ONLY.

umm..then it's bcoz the lining is starting to break down :P sorry my bad :oops: actually i saw the answer as ovulation...first i thought it is menstruation..but then i thought that way...