MIMO antenna configuration is usually expressed as "M * N", what M and N each means? (Select two Answers)

MIMO antenna configuration is usually expressed as "M * N", what M and N each means? (Select two Answers)
A . M means the number of transmitting antennas
B . N means the number of transmitting antennas
C . N means the number of receiving antennas
D . M means the number of receiving antennas

Answer: A C