Which of the Following Is the Biggest Challenge with Selecting

Which of the following is the biggest challenge with selecting a site for a reserve?

A) Making a proper selection is difficult because the environmental conditions of sites can change quickly.
B) Keystone species are difficult to identify in potential reserve sites.
C) Only lands that are not useful to human activities are available for reserves.
D) Most of the best sites are inaccessible by land transportation, so making roads to them is often prohibitively expensive.