Question

what are 1a, 3b, and 7a examples of on the periodic table?
○ groups.
○ numbers
○ periods
○ rows

Explanation:

On the periodic table, groups are vertical columns of elements. The labels like 1A, 3B, 7A represent these vertical columns (groups). Periods are horizontal rows (labeled by numbers like 1, 2, etc.), "numbers" is too vague, and "rows" are periods. So 1A, 3B, 7A are groups.

Answer:

A. groups.