23.
Your Answer is
Correct Answer is D
Explanation
The former sentence "Visitors can not only drop what they don't need, but also take away what they need", and the second sentence "The pile of wood chips, which is very suitable for garden mulch, can also be found there." Nevertheless "however", on the contrary "on the contrary", in spite of it all "despite". There is no turning and concession relationship between the two sentences, so there is no need for correlative words, and D is correct.