<span>contain a decreasing number of organisms going from the bottom to the top of the pyramid</span>
C because you are reflecting, like the translation in math.
A contractile vacuole works just the same as the name suggests, in that it expands and contracts. The point of the contractile vacuole is to pump water out of the cell through a process called osmoregulation, the regulation of osmotic pressure. It occurs in freshwater protists, but mainly in the kingdom Protista as a whole.
A cat thinks that you are opening a can of cat food when it hears the can opener.
A dog shying away from a magazine or newspaper, if the owner has used one to swat the dog as punishment.
If humans are ok to use as examples, A child learning that a stove is hot by touching it.
Well, when chromosomes cross over during prophase 1 of meiosis, the homologous pair of chromosomes exchange segments of DNA which happen to be the genes. So the significance of crossing-over during prophase 1 is that it adds genetic variation to sexually reproducing organisms.
