(In thousands)
2019
2018
2017
Stock-based compensation expense included in cost of sales
$369
$418
$379
Selling, general and administrative expense
3,889
3,989
4,063
Research and development expense
2,704
2,748
2,991
Stock-based compensation expense included in operating expenses
6,593
6,737
7,054
Total stock-based compensation expense
6,962
7,155
7,433
Tax benefit for expense associated with non-qualified options, PSUs, RSUs and restricted stock
(1,659)
(1,432)
(1,699)
Total stock-based compensation expense, net of tax
$5,303
$5,723
$5,734
note 4 – stock-based compensation .
stock incentive program descriptions .
in january 2006 the board of directors adopted the adtran inc .
2006 employee stock incentive plan -the “ 2006 plan ” which authorized 13 .
0 million shares of common stock for issuance to certain employees and officers through incentive stock options and non-qualified stock options stock appreciation rights rsus and restricted stock .
the 2006 plan was adopted by stockholder approval at our annual meeting of stockholders held in may 2006 .
options granted under the 2006 plan typically become exercisable beginning after one year of continued employment normally pursuant to a four-year vesting schedule beginning on the first anniversary of the grant date and had a ten-year contractual term .
the 2006 plan was replaced in may 2015 by the adtran inc .
2015 employee stock incentive plan -the “ 2015 plan ” .
expiration dates of options outstanding as of december 31 2019 under the 2006 plan range from 2020 to 2024 .
in january 2015 the board of directors adopted the 2015 plan which authorized 7 .
7 million shares of common stock for issuance to certain employees and officers through incentive stock options and non-qualified stock options stock appreciation rights psus rsus and restricted stock .
the 2015 plan was adopted by stockholder approval at our annual meeting of stockholders held in may 2015 .
psus rsus and restricted stock granted under the 2015 plan reduce the shares authorized for issuance under the 2015 plan by 2 .
5 shares of common stock for each share underlying the award .
options granted under the 2015 plan typically become exercisable beginning after one year of continued employment normally pursuant to a four-year vesting schedule beginning on the first anniversary of the grant date and have a ten-year contractual term .
expiration dates of options outstanding as of december 31 2019 under the 2015 plan range from 2025 to 2026 .
our stockholders approved the 2010 directors stock plan -the “ 2010 directors plan ” in may 2010 under which 0 .
5 million shares of common stock have been reserved for issuance .
this plan replaced the 2005 directors stock option plan .
under the 2010 directors plan the company may issue stock options restricted stock and rsus to our non-employee directors .
stock awards issued under the 2010 directors plan become vested in full on the first anniversary of the grant date .
options issued under the 2010 directors plan had a ten-year contractual term .
all remaining options under the 2010 directors plan expired in 2019 .
the following table summarizes stock-based compensation expense related to stock options psus rsus and restricted stock for the years ended december 31 2019 2018 and 2017 which was recognized as follows : .

Question: Which year did all the remaining options under the 2010 Directors Plan expire?
Answer: 2019