american tower corporation and subsidiaries notes to consolidated financial statements 2014 ( continued ) pro forma disclosure 2014the company has adopted the disclosure-only provisions of sfas no .
123 , as amended by sfas no .
148 , and has presented such disclosure in note 1 .
the 201cfair value 201d of each option grant is estimated on the date of grant using the black-scholes option pricing model .
the weighted average fair values of the company 2019s options granted during 2004 , 2003 and 2002 were $ 7.05 , $ 6.32 , and $ 2.23 per share , respectively .
key assumptions used to apply this pricing model are as follows: .

2004
2003
2002
Approximate risk-free interest rate
4.23%
4.00%
4.53%
Expected life of option grants
4 years
4 years
5 years
Expected volatility of underlying stock (the Company Plan)
80.6%
86.6%
92.3%
Expected volatility of underlying stock (ATC Mexico and ATC South America Plans)
N/A
N/A
N/A
Expected dividends
N/A
N/A
N/A

voluntary option exchanges 2014in february 2004 , the company issued to eligible employees 1032717 options with an exercise price of $ 11.19 per share , the fair market value of the class a common stock on the date of grant .
these options were issued in connection with a voluntary option exchange program entered into by the company in august 2003 , where the company accepted for surrender and cancelled options ( having an exercise price of $ 10.25 or greater ) to purchase 1831981 shares of its class a common stock .
the program , which was offered to both full and part-time employees , excluding the company 2019s executive officers and its directors , called for the grant ( at least six months and one day from the surrender date to employees still employed on that date ) of new options exercisable for two shares of class a common stock for every three shares of class a common stock issuable upon exercise of a surrendered option .
no options were granted to any employees who participated in the exchange offer between the cancellation date and the new grant date .
in may 2002 , the company issued to eligible employees 2027612 options with an exercise price of $ 3.84 per share , the fair market value of the class a common stock on the date of grant .
these options were issued in connection with a voluntary option exchange program entered into by the company in october 2001 , where the company accepted for surrender and cancelled options to purchase 3471211 shares of its class a common stock .
the program , which was offered to both full and part-time employees , excluding most of the company 2019s executive officers , called for the grant ( at least six months and one day from the surrender date to employees still employed on that date ) of new options exercisable for two shares of class a common stock for every three shares of class a common stock issuable upon exercise of a surrendered option .
no options were granted to any employees who participated in the exchange offer between the cancellation date and the new grant date .
atc mexico holding stock option plan 2014the company maintains a stock option plan in its atc mexico subsidiary ( atc mexico plan ) .
the atc mexico plan provides for the issuance of options to officers , employees , directors and consultants of atc mexico .
the atc mexico plan limits the number of shares of common stock which may be granted to an aggregate of 360 shares , subject to adjustment based on changes in atc mexico 2019s capital structure .
during 2002 , atc mexico granted options to purchase 318 shares of atc mexico common stock to officers and employees .
such options were issued at one time with an exercise price of $ 10000 per share .
the exercise price per share was at fair market value as determined by the board of directors with the assistance of an independent appraisal performed at the company 2019s request .
the fair value of atc mexico plan options granted during 2002 were $ 3611 per share as determined by using the black-scholes option pricing model .
as described in note 10 , all outstanding options were exercised in march 2004 .
no options under the atc mexico plan were granted in 2004 or 2003 , or exercised or cancelled in 2003 or 2002 , and no options were exercisable as of december 31 , 2003 or 2002 .
( see note 10. ) .

Question: Analyse this data from a financial earnings document. based on the black-scholes option pricing model what was the percent of the change in approximate risk-free interest rate from 2003 to 2004
Choices:
0.0007
0.0544
0.0575
0.2656
4.23
Correct choice index: 2