Read the following context and choose the best option to answer the question.
Context: Every other kind of sport had people outside ; hikers , bikers , boaters , rollerbladers , you name it . So where did the rock climbers go ? Now that there are indoor climbing walls , has everybody who likes to climb moved indoors ?
Question: Why do n't rock climbers climb outside anymore ?
Options: 
A. None of the above choices .
B. They want to get more fresh air .
C. There is nothing to climb outside .
D. Indoor rock climbing walls are more convenient .


D