Engines that operate with rotors instead of pistons or cylinders are seldom used in modern automobiles because a. they don't produce enough power. b. they produce dangerous emissions and aren't fuel efficient. c. they can be highly explosive. d. the batteries are heavy and expensive.

B is the correct option.

Explanation:

There are various factors which makes a rotary engine inefficient for use in automobiles. Some of them include Lubrication, Efficiency and cooling, Emissions and fragility. The emissions of rotary engine are in form of excessive carbon dioxide and hydrocarbons. It is one of main hindrance of its widespread acceptance. The engines are also fragile and can't tolerate oiling, cooling and fuel system failures like piston engines do. It also has more than average fuel consumption since the combustion chamber is long and the ares in contact of the combustion mixture leads to loss of energy.

trade regulation is a field of law, often bracketed with antitrust (as in the phrase “antitrust and trade regulation law”), including government regulation of unfair methods of competition and unfair or deceptive business acts or practices. antitrust law is often considered a subset of trade regulation law. franchise and distribution law, consumer protection law, and advertising law are sometimes considered parts of trade regulation law.

in general the theory of economics says that the more open to trade a country is the better   the result for the same country and the whole economies.

in real life countries impose taxes, quotas, etc which is not so good for the economies.

this kind of commercial war between the usa and china is slowing down the growth of the whole world so we can say that it is a draw back of trade regulations. and the free trade agreements are benefits of trade regulations.