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Any of the 2 following statements should be adequate. 

The a) is more likely to be over 18 hours as small samples are more prone to fall further than the true mean.

The higher the number of m. phones the greater the sample. Therefore the closer we are to the true mean. 

If the population follows a normal distribution, the sample mean is also normal with the same mean and a variance which is inversely proportional to the sample size. In case of sample (a), the variance would be inversely proportional to 10, while it would be inversely proportional to 50 for sample (b). Thus, sample (a) has greater variance (i.e. deviation from the mean) and would have a higher probability that the mean battery time is greater than 18 hours, as compared to sample (b).

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