完形填空 Cars can pollute the air,but it can't s----

原文就在这段话里,打括号的就是要填的单词~
Although cars are a main pollutant of the environment,it hasn't （stopped） designers1 from making new cars and showing their ideas to the public。（Maybe） the cars of the future will not pollute the air as much。For the moment,we don't know if the Magic2Tricycle will be good to the environment,but we are （sure） that it looks great and we would like to see it on the（ roads）。
It's called the Magic Tricycle （because） it has three wheels for usual driving on the road; but,when you find yourself in busy （traffic）,the Magic Tricycle works its magic and the left wheel （lifts) into the air so that the Magic Tricycle becomes a motorcycle with two wheels。With this design,Seyyed Javad Ghaffarian (won) the 2008 Car News Design Contest and many people were very pleased with this new design。"One of the (biggest) difficulties with this design was how to make it attractive in (both) its two forms。" he said。