Which of the following are isobars ?

1. $^{12}$6\text{C} and $^{14}6\text{C}$
2. $^{16}$8\text{O} and $^{23}{11}\text{Na}$
3. $^{23}${11}\text{Na} and $^{22}{11}\text{Mg}$
4. $^{23}${11}\text{Na} and $^{23}{12}\text{Mg}$

Which of the following is not a type of equilibrium ?

1. Static
2. Dynamic
3. Neutral
4. Rolling

Absolute unit of work is

1. joule
2. erg
3. Both (a) and (b)
4. None of these

The effective diameter of the circular outline of a spherical lens is known as

1. focal length
2. principal focus
3. aperture of lens
4. radius of curvature