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A trait that can mask another trait is known as a dominant trait.
Answer: Contractile vacuole of amoeba will shrink/contract when the organism is placed in sea water.
Contractile vacuole is a subcellular structure ( mainly found in protists) that is involved in the regulation of water quantity inside a cell.
As we know that water always flow from a hypotonic ( where solute concentration is less) to a hypertonic ( where solute concentration is more) solution. Sea water is a hypertonic solution as the concentration of salt is very high. Therefore, in this case, water will flow from contractile vacuole to outside of the organism.
Thus, contractile vacuole of amoeba will contract as it will lose water when placed in sea water.
Because if we're simultaneously moving/manipulating two different variables it can be very hard to determine why one variable is changing and why the other is changing. While this can be done with more complicated experimental designs, it usually is the case that scientists will only manipulate one variable and not two different ones.