I believe that this is what the explanation should have been.

The answers are: SELECT COUNT_BIG(ALL COlumn_2) and SELECT COUNT_BIG(column_2). COUNT_BIG(*) will count all rows from the table (including duplicate & null rows). COUNT_BIG(DISTINCT column_2) will evaluate distinct non-null values.

ALL is the default and hence COUNT_BIG(column 2) is same as COUNT_BIG(ALL column_2). This evaluates expression for each row in a group and returns the number of nonnull values.

COUNT_BIG is works like COUNT, only difference is that COUNT_BIG returns bigint data type.