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A science question involving some basic maths. Ans key says (1) but I think otherwise. I think the answer should be (3). You could find the mass of ball C by taking Container Z minus Container Y. But you could also find the volume of Object B by taking Container Y minus Container X. The only object you can’t find is object A, since we don’t know how much water he added at first. So ans should be (3) and not (1), or am I missing something?