(a) Twice that of A
   (b) 4 times that of A
   (c) 1⁄4 that of A
   (d) 1⁄2 that of A
Ans: (d)
86.Two thin convex lenses of focal lengths 4 cm and 8 cm are separated by a
   distance of 4 cm in air. The combination will have the focal length? [NDA
   2012]
   (a) 4 cm
   (b) 8 cm
   (c) 12 cm
   (d) 32 cm
Ans: (a)
87.In the isothermal process which of the following remains constant:
   (a) temperature
   (b) heat
   (c) pressure
   (d) density
Ans: (a)
88.In the adiabatic process which of the following remains constant:
   (a) temperature
   (b) heat
   (c) pressure
   (d) volume
Ans: (b)
89.For the adiabatic process ideal gas equation is expressed as :
   (a) pVt = constant
   (b) pVt-1
   (c) pyVt-1 = constant
   (d) any of these
Ans: (a)
90.For the isothermal process ideal gas equation is expressed as :
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