I am trying to write a 500 word essay in response to this prompt:"Why has globalization meant greater dependence of Canada on the United States?...

First, you've made a good effort to get started on your paper. There are a few more research steps you need to take. One important one is to ignore Quora results. It's not a reliable site with answers edited and vetted by experts (answers there can be written by anyone--including people who might not know a lot about the topic). Both the US and Canadian governments publish data concerning volumes of trade. This is a good starting place to get reliable numbers. 

The US government has a webpage showing trade statistics between Canada and the US since 1985. An even more valuable source is Global Affairs Canada, which has pages focused on Canada's trade with all its major global trading partners. Currently, the US takes 76 percent of Canada's merchandise exports, while China accounts for under four percent and the Asia-Pacific region about 10 percent. In terms of imports, trade is more balanced, with the US being the source of approximately half of Canadian imports and China having a 12 percent share. 

For globalization, you might want to start with definition. In general, it is contrasted with national self-sufficiency. Trade between Canada and the US, and NAFTA as a regional trade deal, are both parts of globalization. Canadian exports to the US have risen dramatically since the passage of NAFTA.

This interdependence of Canada and the United States is problematic in that it ties Canada's fortunes to that of the US economy. If the US swings towards a protectionist trade policy or goes through a major recession, Canada would suffer economic consequences. 

There are really two ways Canada can protect itself. One is to ensure that crucial strategic industries thrive in Canada rather than depending on the US. The building of the Canadian Pacific Railroad, subsidies to Air Canada, and attempts to build oil pipelines to the east and west coast are part of such defensive strategies, as are efforts to prevent selling strategic industries to foreign investors. The second way Canada can reduce dependence on the US is to make increased efforts to export to other countries.