discount to brent was narrower in 2013 than in 2012 and 2011 .
as a result of the significant increase in u.s .
production of light sweet crude oil , the historical relationship between wti , brent and lls pricing may not be indicative of future periods .
composition 2013 the proportion of our liquid hydrocarbon sales volumes that are ngls continues to increase due to our development of united states unconventional liquids-rich plays .
ngls were 15 percent of our north america e&p liquid hydrocarbon sales volumes in 2013 compared to 10 percent in 2012 and 7 percent in 2011 .
natural gas 2013 a significant portion of our natural gas production in the u.s .
is sold at bid-week prices , or first-of-month indices relative to our specific producing areas .
average henry hub settlement prices for natural gas were 31 percent higher for 2013 than for 2012 .
international e&p liquid hydrocarbons 2013 our international e&p crude oil production is relatively sweet and has historically sold in relation to the brent crude benchmark , which on average was 3 percent lower for 2013 than 2012 .
natural gas 2013 our major international e&p natural gas-producing regions are europe and e.g .
natural gas prices in europe have been considerably higher than the u.s .
in recent years .
in the case of e.g. , our natural gas sales are subject to term contracts , making realized prices in these areas less volatile .
the natural gas sales from e.g .
are at fixed prices ; therefore , our reported average international e&p natural gas realized prices may not fully track market price movements .
oil sands mining the oil sands mining segment produces and sells various qualities of synthetic crude oil .
output mix can be impacted by operational problems or planned unit outages at the mines or upgrader .
sales prices for roughly two-thirds of the normal output mix has historically tracked movements in wti and one-third has historically tracked movements in the canadian heavy crude oil marker , primarily wcs .
the wcs discount to wti has been increasing on average in each year presented below .
despite a wider wcs discount in 2013 , our average oil sands mining price realizations increased due to a greater proportion of higher value synthetic crude oil sales volumes compared to 2012 .
the operating cost structure of the oil sands mining operations is predominantly fixed and therefore many of the costs incurred in times of full operation continue during production downtime .
per-unit costs are sensitive to production rates .
key variable costs are natural gas and diesel fuel , which track commodity markets such as the aeco natural gas sales index and crude oil prices , respectively .
the table below shows average benchmark prices that impact both our revenues and variable costs: .

Benchmark
2013
2012
2011
WTI crude oil(Dollars per bbl)
$98.05
$94.15
$95.11
WCS(Dollars per bbl)<sup>(a)</sup>
$72.77
$73.18
$77.97
AECO natural gas sales index(Dollars per mmbtu)<sup>(b)</sup>
$3.08
$2.39
$3.68

wcs ( dollars per bbl ) ( a ) $ 72.77 $ 73.18 $ 77.97 aeco natural gas sales index ( dollars per mmbtu ) ( b ) $ 3.08 $ 2.39 $ 3.68 ( a ) monthly pricing based upon average wti adjusted for differentials unique to western canada .
( b ) monthly average day ahead index. .

Question: Analyse this data from a financial earnings document. by what percentage did the aeco natural gas sales index decline from 2011 to 2013?
Choices:
-0.06
-0.18478
24.84511
-6.13333
-0.16304
Correct choice index: 4