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the frequency table shows the results of three surveys that each recorded the number of trees per block for four blocks in a city.

based on the unbiased surveys, estimate the median number of trees per block in the city. round to the nearest whole number.

survey trees per block
#1 {10,16,14,20}
#2 {11,17,15,21}
#3 {18,16,13,13}

a 13

b 15

c 16

d 21