note 9 : stock based compensation the company has granted stock option and restricted stock unit ( 201crsus 201d ) awards to non-employee directors , officers and other key employees of the company pursuant to the terms of its 2007 omnibus equity compensation plan ( the 201c2007 plan 201d ) .
the total aggregate number of shares of common stock that may be issued under the 2007 plan is 15.5 .
as of december 31 , 2015 , 8.4 shares were available for grant under the 2007 plan .
shares issued under the 2007 plan may be authorized-but-unissued shares of company stock or reacquired shares of company stock , including shares purchased by the company on the open market .
the company recognizes compensation expense for stock awards over the vesting period of the award .
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: .

2015
2014
2013
Stock options
$2
$2
$3
RSUs
8
10
9
ESPP
1
1
1
Stock-based compensation
11
13
13
Income tax benefit
(4)
(5)
(5)
Stock-based compensation expense, net of tax
$7
$8
$8

there were no significant stock-based compensation costs capitalized during the years ended december 31 , 2015 , 2014 and 2013 .
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 2015 , 2014 and 2013 are classified as equity .
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 common stockholders 2019 equity or the statement of operations and are presented in the financing section of the consolidated statements of cash flows .
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 .
stock options in 2015 , 2014 and 2013 , 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 .
these awards have no performance vesting conditions and the grant date fair value is amortized through expense over the requisite service period using the straight-line method and is included in operations and maintenance expense in the accompanying consolidated statements of operations. .

Question: Analyse this data from a financial earnings document. at what tax rate was stock-based compensation taxed at in 2018?
Choices:
3.67
0.36
-1.25
0.25
2.75
Correct choice index: 4