Lisa Has Been Widowed Three Times

Lisa has been widowed three times. Her first husband died in 2011, leaving an unused exclusion amount of $3.5 million. The second husband died in 2013, leaving the entire $5.25 million exclusion amount unused. Lisa's third husband died in 2018 with an unused exclusion amount of $4 million. Lisa's DSUE amount is:

A) $3.5 million.
B) $4 million.
C) $5.25 million.
D) $12.5 million.