yes the ans is C.
To find the std deviation, it is necessary to know both the number of terms and the distance of each term from the average.
1. is insufficient , since it only tells us theat the number of terms in both sets is equal.
2. insufficient, since even though the distance between each term is given, the average of the terms will vary when the number of terms is even, from when it is odd.
the statements combined are sufficient. The avg of each set will be the fourth term, and each of the seven terms is 2 away from each term closest to it.