on may 12 , 2017 , the company 2019s stockholders approved the american water works company , inc .
2017 omnibus equity compensation plan ( the 201c2017 omnibus plan 201d ) .
a total of 7.2 million shares of common stock may be issued under the 2017 omnibus plan .
as of december 31 , 2017 , 7.2 million shares were available for grant under the 2017 omnibus plan .
the 2017 omnibus plan provides that grants of awards may be in any of the following forms : incentive stock options , nonqualified stock options , stock appreciation rights , stock units , stock awards , other stock-based awards and dividend equivalents , which may be granted only on stock units or other stock-based awards .
following the approval of the 2017 omnibus plan , no additional awards are to be granted under the 2007 plan .
however , shares will still be issued under the 2007 plan pursuant to the terms of awards previously issued under that plan prior to may 12 , 2017 .
the cost of services received from employees in exchange for the issuance of stock options and restricted stock awards is measured based on the grant date fair value of the awards issued .
the value of stock options and rsus awards at the date of the grant is amortized through expense over the three-year service period .
all awards granted in 2017 , 2016 and 2015 are classified as equity .
the company recognizes compensation expense for stock awards over the vesting period of the award .
the company stratified its grant populations and used historic employee turnover rates to estimate employee forfeitures .
the estimated rate is compared to the actual forfeitures at the end of the reporting period and adjusted as necessary .
the following table presents stock-based compensation expense recorded in operation and maintenance expense in the accompanying consolidated statements of operations for the years ended december 31: .

2017
2016
2015
Stock options
$1
$2
$2
RSUs
9
8
8
Nonqualified employee stock purchase plan
1
1
1
Stock-based compensation
11
11
11
Income tax benefit
(4)
(4)
(4)
Stock-based compensation expense, net of tax
$7
$7
$7

there were no significant stock-based compensation costs capitalized during the years ended december 31 , 2017 , 2016 and 2015 .
the company receives a tax deduction based on the intrinsic value of the award at the exercise date for stock options and the distribution date for rsus .
for each award , throughout the requisite service period , the company recognizes the tax benefits , which have been included in deferred income tax assets , related to compensation costs .
the tax deductions in excess of the benefits recorded throughout the requisite service period are recorded to the consolidated statements of operations and are presented in the financing section of the consolidated statements of cash flows .
stock options there were no grants of stock options to employees in 2017 .
in 2016 and 2015 , the company granted non-qualified stock options to certain employees under the 2007 plan .
the stock options vest ratably over the three-year service period beginning on january 1 of the year of the grant and have no performance vesting conditions .
expense is recognized using the straight-line method and is amortized over the requisite service period. .

Question: Analyse this data from a financial earnings document. at what tax rate was stock-based compensation being taxed at during the years 2015 , 2016 and 2017?
Choices:
-0.36364
1.81818
-2.75
0.63636
-15
Correct choice index: 0