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
flagBelarus
47%
9%
33%
4%
21
flagthe United States
46%
19%
20%
12%
315
flagSwitzerland
46%
15%
7%
30%
13
flagAustria
45%
16%
25%
12%
31
flagIndia
44%
25%
16%
13%
144
flagCanada
44%
16%
33%
5%
18
flagUkraine
43%
16%
25%
15%
124
flagRepublic of Ireland
43%
12%
25%
18%
16
flagthe Philippines
43%
18%
25%
12%
16
flagthe United Kingdom
42%
21%
20%
15%
115
flagPoland
41%
20%
22%
15%
13293
flagRussia
41%
20%
25%
10%
82
flagUnknown
40%
18%
27%
13%
22
flagGermany
38%
26%
21%
11%
175
flagFrance
33%
25%
16%
25%
12
flagGreece
31%
21%
15%
31%
19
flagKazakhstan
30%
23%
23%
23%
13
flagAustralia
21%
21%
26%
30%
23