Question: What is the maximum number of secondary cells that can be aggregated for transmissions in multiple cells?
Option 1: 15
Option 2: 31
Option 3: 32
Option 4: 64
Explanation: According to the specification, up to 31 secondary cells can be used in addition to the primary cell for transmissions in multiple cells. This allows for multi-cell aggregation with different frame structures being used in the different serving cells.