单选题 According to the passage, the fireball which emits the initial GRBs differs from the stage in which it produces aftershocks in that

• (A) the former lacks the accelerated velocity of the latter
• (B) the former is easier to observe than the latter
• (C) the former has higher photon density than the latter
• (D) the former contains a greater number of positrons than the latter
• (E) the former is in a state of expansion while the latter is in a state of contraction