What are two important considerations when using 2- or 4-channel 15216 OADMs？（）

• A、 All "Add" channels require uniform power levels.
• B、 The flexibility of wavelengths per OADM is limited.
• C、 It is easier to balance "Pass-Through" and "Add" channels.
• D、 There is lower pass-through loss using multi-channel OADMs.
• E、 You can extract a single channel without dropping other channels.