"Howdy, Modi!" trends on Twitter after Trump's "India Filthy" remark

US President Donald Trump's remark about "filthy air" in India - made during a presidential debate early Friday morning - has provoked myriad reactions on Twitter, with views ranging from somber acceptance of a pollution problem in the country to digs at Prime Minister Narendra Modi over his "great friendship" with American President and last year's "Howdy, Modi!" event.