[单选题]
4. What causes decalcification of corals gdpds wm1:+2p g0op2pleadina b (-7qz0 - a6j husfpjy e3b5*s3k3oodg7kvsg to collapse of coral ecosystems 36-*k7se7hfg3od50y(o bz s qa- vuk3p jjsba?
A. Coral bleaching.
B. Ocean acidification.
C. Global warming.
D. The El Nino effect.