Find the Error — Email Builder Skips a LOWER

A communications analyst's CONCAT-built email column is producing mixed-case last names — "sarah.Smith@acme.com" instead of "sarah.smith@acme.com", "david.Martinez@hooli.net" instead of "david.martinez@hooli.net". The downstream SMTP server is case-insensitive on the local part so the emails deliver, but the customer database does case-sensitive deduplication and the mixed-case rows show up as separate contacts.

Their formula in column D:

=LOWER(A2) & "." & B2 & "@" & C2

The bug is hiding in plain sight. The first name (column A) gets folded with LOWER, but the last name (column B) is concatenated raw — so any non-lowercase letter in column B leaks into the output.

The fix: wrap column B in LOWER too, or wrap the whole concatenation:

=LOWER(A2) & "." & LOWER(B2) & "@" & C2 or equivalently =LOWER(A2 & "." & B2 & "@" & C2)

The second form is more defensive — any column added to the chain inherits the case-folding automatically.

The data at A2:C13 has 12 employees with intentionally inconsistent casing across the First and Last name columns (some lowercase, some Mixed Case, some ALL CAPS). The Domain column is already lowercase in every row.

Your task:

1. In column D (Email), write a corrected formula that returns the all-lowercase email for each row.
2. The formula must drag-fill from D2 → D13.

Graded cells: D2 (sarah + Smith + acme.com → sarah.smith@acme.com), D5 (DAVID + Martinez + hooli.net → david.martinez@hooli.net), D8 (lisa + WONG + sterlingcooper.agency → lisa.wong@sterlingcooper.agency).