Take two balloons inflate them. Rub both with your hair. Bring them to closer to each other. Then they 

i when lenses are kept in contact then 1/f = 1/f1 + 1/f2 f = f1f2 / f1 + f2 ii. When kept distant apart, 1/f = 1/f1 +1/f2 - d/f1f2

Correct option is (A) 1 If two lines intersect each other then they have exactly one common point.

Correct option is  D) adjacent

B) electrostatic force

Correct option is (D) \(7\,\frac{1}{2}\) hrs \(\because\) Time taken to fill a \(50\,l\) of tank = 5 hours \(\therefore\) Time taken to fill a \(75\,l\) of tank \(=\frac5{50}\times75\;hours\) \(=\frac{75}{10}\;hours\) = 7.5 hours = 7 hours + \(\frac12\) hours = \(7\frac12\) hours

Correct Answer - D Argon is unsuitable for filling up weather observation balloons because it is heavier than air.

Correct Answer - `0004`

Divisional structure