Explain why
Atmospheric pressure at a height of about 6km decreases to nearly half to its value at the sea level, through the height of the atmospheric is more than 100 km.

A ground receiver station is receiving a signal at 100 MHz, transmitted from a ground transmitter at a height of 300 m located at a distance of 100 km. It is coming via (Given the value of radius of the earth = 6400 km and maximum electron density, N_(max) = 10^12 m^-3 ).

In a sub - stage lunch of a satellite, the first stage brings the satellite to a height of 150 km and the second stage gives it the necessary critical speed to put . It in a circular orbit around the Earth , which stage requires more expenditure of fuel ? (Given , M_(E)=6.0xx10^(24)kg,R=6.4xx10^(6)m,G-6.67xx10^(-11)Nm^(2)kg^(-2)).

Assertion. The atmospheric concentration of CO_(2) at which photosynthesis just compensates for resoiration is referred to as CO_(2) compensation point. Reason. CO_(2) -compensation point is reached when the amount of CO_(2) uptake is less than that generated through respiration because the level of CO_(2) is more than that required for achieving CO_(2) -compensation point.