Before the North American Free Trade Agreement (NAFTA) was signed, there was a great discussion at all levels. Congress, the unions, the public, the President, offered their views. The Agreement finally entered into force in 1994. Free trade agreements do not affect everyone equally; That is why their signatures involve "heated" discussions; to the point that the current President of the United States, Mr. Donald Trump, proposed the renegotiation not only of NAFTA, but of all the trade agreements signed, including those it has with the European Economic Community and challenged China by imposing tariffs on imports of goods and services from that country. In December 2018, the United States, Mexico and Canada signed an agreement: the United States, Mexico and Canada Agreement (USMCA), which replaced NAFTA.

a) What did the signing of the NAFTA treaty mean?

b) What did the replacement of the USMCA mean?

Note: Please add Reference.