Why are mobile phones popular in China, according to the speaker a. They are considered as fashionable items. b. They keep people closer to each other. c. They are more convenient to users than fixed phones. d. They are considered time-saving devices.

闻一多在《诗镌》第七期上发表了诗论《诗的格律》，提出了著名的“三美”主张。请简要说明闻一多“三美”主张的主要内容。