Which of the lines on the right, is the continuation of the black line on the left?

red one
blue one
none of them
both of them
This is known as Poggendorff illusion, first described in 1860. Many detailed studies of the illusion point to its principal cause: acute angles in the figure are seen by viewers as expanded.
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Answers by country

red one
blue one
none of them
both of them
answers
flagthe Netherlands
70%
10%
10%
10%
10
flagAustria
48%
13%
27%
10%
29
flagthe United States
45%
19%
21%
12%
307
flagIndia
45%
24%
16%
12%
140
flagBelarus
45%
10%
35%
5%
20
flagCanada
44%
16%
33%
5%
18
flagUkraine
43%
16%
25%
15%
120
flagRepublic of Ireland
43%
12%
25%
18%
16
flagthe Philippines
43%
18%
25%
12%
16
flagthe United Kingdom
42%
21%
20%
15%
113
flagPoland
41%
20%
22%
15%
13195
flagGermany
41%
25%
16%
13%
130
flagRussia
41%
21%
24%
11%
78
flagSwitzerland
41%
16%
8%
33%
12
flagUnknown
40%
18%
27%
13%
22
flagKazakhstan
33%
16%
25%
25%
12
flagFrance
33%
25%
16%
25%
12
flagGreece
31%
21%
15%
31%
19
flagAustralia
21%
21%
26%
30%
23