An average student remembers about 50 percent of a ten-minute lecture when tested (2)immediately; and 25 percent of same lecture when testd fifty-five hours later.

(2)
a)No change
b)immediately -- and
c)immediately and

I chose answer c) since I didn't think a comma was necessary in this case.
thanks

Where are the instructions of what you are eactly to do"

Sra

You're right. The comma is unnecessary.

Your choice of answer c) is correct. In this case, the phrase "immediately and" does not require a comma before it. The phrase "immediately and" is used to connect two different points in time when the student's memory is tested - "immediately" and "fifty-five hours later". Since there is no need to separate these two points with a comma, option c) is the most appropriate choice. Well done!