Based on the following input and output, which field was the group by action applied to with Summarize tool?

$\backslash$Sigma

Input:

Record

$2456$

$331$

Score

$2$

First PurchaseDate

$2014-08-21$

T

$2014-04-01$

$1$

$2014-12-21$

$45$

$2014-10-01$

Northeast

$22$

$2012-05-05$

$51$

$2013-09-06$

West

$2013-09-06$

$62$

$2014-01-26$

Midwest

a

$2014-01-26$

Output:

Record

Region

Max Join Date

$1$ Midwest

$2014-12-21$

$2$ Northeast

$2014-10-01$

$3$ South

$2014-08-21$

$4$ West

$2013-09-06$

A$.$ JoinDate

B$.$ Score

C$.$ Region

D$.$ CustomerID