50. Which sentence is the best "topic sentence" for the following paragraph?"__________.People with a specially-made headset work out in a "black box". They can control the intensity of the exercise by pushing buttons from the VR controller. They can also compete against others from simulated fight scenes in sports games. This VR system has created a safe environment to prevent injuries during exercises. The concept of getting fit in anywhere at any time can be practiced by using VR. VR has definitely changed the way people exercise."(A) It is of great importance to create virtual reality systems to promote fitness training for adults.(B) Using a black box in virtual reality training has made the exercises less dangerous.(C) A virtual reality company has developed a cutting-age system for gym users.(D) People find it interesting to have regular exercises at the gym by using virtual reality equipment.