Which statements are correct for Doppler shifts involving reflected sound?

Select all that apply

• Option A

  Reflection from a moving wall can involve two Doppler shifts.

• Option B

  A source moving towards a stationary wall can hear its echo at a frequency higher than its emitted frequency.

• Option C

  For a source moving towards a stationary wall, fₑ = f×(v+u)/(v-u).

• Option D

  Direct and reflected sounds can produce audible beats if their received frequencies differ slightly.