Riddler - Counting Trees

Walking down a street, a man counts 10 trees on his right. On your return, count 10 trees on your left. How many trees has he seen in total?

a) 10
b) 20
c) 30
d) 40


In total he saw 10 trees, because the 10 he saw on the way back are the same ones he saw on the way, as he is walking on the same street.