Niagara Falls ________(be) a famous waterfall in Canada.

主要给我说明为什么

is

is 因为这个尼亚加拉大瀑布是个一个瀑布，所以用单数形式。

因为前面那个是指加拿大的尼亚加拉大瀑布，是一个专有名词，做单数用。如Niagara Falls comprises three distinct cataracts. 但是要讲里边的瀑布群时，就可以用复数了。如Niagara Falls comprises three distinct cataracts. The tallest are the American and Bridal Veil falls on the American side, separated by tiny Luna Island and plunging over jagged rocks in a 180ft drop; the broad Horseshoe Falls which curve their way over to Canada are probably the most impressive. They date back a mere twelve thousand years, when the retreat of melting glaciers allowed water trapped in Lake Erie to gush north to Lake Ontario. Back then the falls were seven miles downriver, but constant erosion has cut them back to their present site. The falls are colorfully lit up at night, and many say they're most beautiful in winter, when the grounds are covered in snow and the waters turn to ice.