The cause for the scandal was the discovery of bribery in the cabinet and the government regarding oil trade. One effect was that it proved that cabinet members are not all powerful and can be arrested and indicted just like anyone. Another effect was the Supreme Court's ruling in McGrain v. Daugherty which explicitly established that Congress had the power to compel testimony. A third effect was that the government was given back the oil fields that were leased and they stayed in US ownership until 2015 when it was legally sold.
To my knowledge, the correct answer among all of the given choices is C.) Steam. If I'm not mistaken, this was the time that more trains came into use.
I hope this is the answer you were looking for and that it helps!! :)
