3pi/7 < pi/2 because 3/7 < 1/2, and pi/2 is a right angle. Conclusion: the angle opposite side a is an acute angle. In this situation the triangle could be a right triangle, in which case C would be true, but it does not have to be a right triangle, so don´t choose C. Similarly, it could be an acute triangle, in which case B would be true, but it does not have to be, so don´t choose B. Also, A says the angle opposite side a is obtuse, which is false. So don´t choose A. That leaves D, which says the angle opposite side a is acute, which we know is true. So the answer is D. b^2 + c^2 > a^2
