Five samples were taken of a population of four-year-old children.

This question tests weighted averages. Because the sample sizes are different, you cannot simply average the mean heights. Instead, multiply each sample mean by its sample size, add the results, and divide by the total number of children.

Total children = 20 + 20 + 25 + 70 + 20 = 155

Total height = (20×95) + (20×100) + (25×100) + (70×110) + (20×105)

= 1,900 + 2,000 + 2,500 + 7,700 + 2,100

= 16,200

Mean height = 16,200 ÷ 155 ≈ 104.5 cm

Therefore, option A is correct. The NEX exam frequently tests understanding that larger samples should carry more weight when calculating population estimates.

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