Complex sentence: The Prius is the top selling hybrid car in the U.S. market with 1 million units sold by April 2011, and cumulative sales of the Prii in Japan reached the 1 million mark in August 2011.
Split into simpler sentences: The Prius is the top selling hybrid car in the U.S. market, surpassing the 1 million milestone in April 2011. Cumulative sales of the Prii in Japan reached the 1 million mark in August 2011.