03·GroupBy and aggregations

What does df.groupby(["a", "b"])["c"].sum() do?

• Sums c for every unique (a, b) pair. The result is a Series with a (a, b) MultiIndex. .unstack("b") turns the second index level into columns.
• Raises an error — only one key allowed.
• Sums all columns, ignoring c.
• Returns a DataFrame without summing.