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Let’s mark the babies mass as B_1, B_2, B_3 with the 1 being the lightest.

B_1 = 1.8kg from the text.

The range of the masses means the difference between the heaviest and the lightest baby:

B_3 - B_1 = 0.7kg \to B_3 = 1.8kg +0.7kg = 2.5kg

The mean is calculated like: \frac{B_1 + B_2 + B_3}{3} =\frac{1.8kg + B_2 + 2.5kg}{3} =2.1kg \to B_2 + 4.3kg = 6.3kg \to B_2 = 6.3kg - 4.3kg = 2kg.

So the answer to this question is **(D)** 2kg

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