In the Book of Daniel, what specific decree of Darius the Mede led to Daniel being thrown into the lions' den?

A A decree forbidding prayer to any god or man except Darius for thirty days. B A decree mandating the worship of the golden image set up by Nebuchadnezzar. C A decree requiring all wise men to interpret a dream that Darius had. D A decree demanding all citizens contribute to the rebuilding of Babylonian temples.