WASHINGTON — President Trump again said Tuesday he wanted Canada to be America’s 51st state — but was rebuffed by newly-elected Prime Minister Mark Carney in a tense exchange in the Oval Office. “I still believe that, but, you know, it takes two to tango,” Trump told reporters — saying too that Carney could offer nothing to change his mind about new tariffs on America’s northern neighbor. Trump later gave more extensive commentary on a possible North American political union, calling the US-Canada border an “artificial line” and saying it would lead to a “massive tax cut” for them. “When...