Two identical spheres move in opposite directions with speeds $v_1$ and $v_2$ and pass behind an opaque screen,where they may either cross without touching (Event $1$) or make an elastic head-on collision (Event $2$).

• A
  We can never make out which event has occurred.
• B
  We cannot make out which event has occurred only if $v_1 = v_2$.
• C
  We can always make out which event has occurred.
• D
  We can make out which event has occurred only if $v_1 = v_2$.