Which of the following time measuring devices is most precise?
$(A)$ $A$ wall clock
$(B)$ $A$ stop watch
$(C)$ $A$ digital watch
$(D)$ An atomic clock
Give reason for your answer.

Precise measurements of physical quantities are a need of science. For example,to ascertain the speed of an aircraft,one must have an accurate method to find its positions at closely separated instants of time. This was the actual motivation behind the discovery of radar in World War $II$. Think of different examples in modern science where precise measurements of length,time,mass,etc.,are needed. Also,wherever you can,give a quantitative idea of the precision needed.

What is the approximate dimension (thickness) of a molecule of oleic acid?

The nearest star to our solar system is $4.29$ light years away. How much is this distance in terms of parsecs? How much parallax would this star (named Alpha Centauri) show when viewed from two locations of the Earth six months apart in its orbit around the Sun?

Given below are densities of some solids and liquids. Give rough estimates of the size of their atoms.

Substance	Atomic Mass $(u)$	Density $(10^3\,kg\,m^{-3})$
Carbon (diamond)	$12.01$	$2.22$
Gold	$197.00$	$19.32$
Nitrogen (liquid)	$14.01$	$1.00$
Lithium	$6.94$	$0.53$
Fluorine (liquid)	$19.00$	$1.14$

Match List-$I$ (Event) with List-$II$ (Order of the time interval for happening of the event) and select the correct option from the options given below the lists:

List-$I$	List-$II$
$(1)$ Rotation period of earth	$(i)$ $10^5\, s$
$(2)$ Revolution period of earth	$(ii)$ $10^7\, s$
$(3)$ Period of light wave	$(iii)$ $10^{-15}\, s$
$(4)$ Period of sound wave	$(iv)$ $10^{-3}\, s$