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
60%
10%
10%
20%
20
flagthe Netherlands
57%
7%
7%
28%
14
flagthe United States
45%
19%
21%
12%
339
flagIndia
45%
22%
16%
13%
159
flagthe Philippines
43%
18%
25%
12%
16
flagthe United Kingdom
42%
21%
20%
15%
128
flagBelarus
42%
11%
38%
3%
26
flagCanada
42%
23%
28%
4%
21
flagPoland
41%
20%
22%
15%
13947
flagGermany
41%
25%
20%
10%
273
flagUkraine
41%
16%
26%
15%
153
flagRepublic of Ireland
41%
11%
29%
17%
17
flagAustria
40%
20%
23%
16%
55
flagUnknown
40%
18%
27%
13%
22
flagFrance
38%
23%
15%
23%
13
flagRussia
36%
19%
28%
13%
139
flagGreece
31%
21%
15%
31%
19
flagKazakhstan
30%
30%
25%
15%
20
flagUzbekistan
23%
7%
30%
38%
13
flagAustralia
20%
20%
28%
28%
25