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
flagSwitzerland
57%
9%
14%
19%
21
flagthe Netherlands
57%
7%
7%
28%
14
flagthe United States
46%
19%
20%
12%
347
flagIndia
45%
22%
17%
13%
160
flagRepublic of Ireland
44%
11%
27%
16%
18
flagthe Philippines
43%
18%
25%
12%
16
flagthe United Kingdom
42%
21%
20%
15%
129
flagBelarus
42%
11%
38%
3%
26
flagCanada
42%
23%
28%
4%
21
flagPoland
41%
20%
22%
15%
13987
flagUkraine
41%
16%
26%
15%
153
flagGermany
40%
25%
20%
11%
277
flagUnknown
40%
18%
27%
13%
22
flagAustria
39%
19%
25%
16%
56
flagFrance
38%
23%
15%
23%
13
flagRussia
36%
19%
29%
13%
140
flagGreece
30%
20%
15%
35%
20
flagKazakhstan
30%
30%
25%
15%
20
flagAustralia
23%
19%
26%
26%
26
flagUzbekistan
23%
7%
30%
38%
13