A bit confused in one question of Mathematics of AILET 2016

I was solving AILET 2016 (Mathematics) for posting solutions to this website. I came to this question (No. 143), “In a garden there are 8 rows and 10 columns of papaya tree. The distance between the two trees is 2 m and a distance of 1 m is left from all sides of boundary of the garden. The length of the garden is: (a) 24 m (b) 14 m (c) 20 m (d) 18 m”. The official answer as provided by NLU Delhi is option (d).

Though the question has been solved here, I am a bit confused as my answer is option (c). Can anybody tell me where I went wrong?