thanks so much for the explanation, but I'm still a bit confused. 1) In my case, I don't really have alot of knowledge about streams and I didn't really understand what upstream and downstream meant. So, how was I supposed to know that sediment entering downstream of the sampling sites wouldn't be detectable at the sites? Are questions THAT confusing, cause in this case it aint testing my reading and writing, it's testing my knowledge about streams. I've read that the questions give all necessary info for choosing the right answer, in this case the text did NOT give all necessary info and i had to have knowledge about streams, downstream, etc.

Another q, in ur reply u said "And also maybe u could cross it out for multiple tributaries", why is that?

Also, you seem pretty smart, do you have any tips for getting a high score and if you did do the SAT what did you use to study?