Answer:
It can be multiple ( B or D ).
The answer can be B because the gold was first discovered by James Marshall. Then he told John Stutter who then told lots of people about it. John telling everyone had an effect ( over 300,000 came to find the gold ).
Another answer is D because it tells you it all in the events it happened in. First James Marshall finding the gold, next him telling John Stutter, Stutter telling everyone and finally people coming to find the gold.
I'm sorry if I made a mistake but these are the two possible answers I have come up with. :)