Analyse the following observation table showing variation of image distance (a) with object distance (u) in case of a convex lens and answer the questions that follow, without doing any calculations :

S. No Object distance u(cm) Image distance v(cm)
1 -90 +18
2 -60 +20
3 -30 +30
4 -20 +60
5 -18 +90
6 -10 +100

Write the serial number of that observation which is not correct. How did you arrive at this conclusion?