B. (k+10f)(k-5f) you can tell in 2 ways. 1. if the second sign is neg then the signs in the factors are going to be one neg and one positive so that knocks out C and D. Then you look at the second term and since the 5 is positive then the 10 has to be positive so you end up with a positive 5kf.
The second way is to FOIL them out. When you check B you get [tex] k^{2} -5kf+10kf- 50f^{2} [/tex] combine your like terms and you have your original expression of [tex] k^{2} +5kf-50 f^{2} [/tex]
Hope that helps
