v = speed of the source of sound or the train towards the listener or switchman = 40 m/s
V = actual speed of sound = 340 m/s
f = actual frequency of sound as emitted from source or the train = 1000 Hz
f' = frequency as observed by the listener or by switchman = ?
Using Doppler's law , frequency observed by a listener from a source moving towards it is given as
f' = V f /(V - v)
inserting the values
f' = 340 x 1000 /(340 - 40)
f' = 340 x 1000/300
The correct answer of this question is : A) Change alternating current into direct current.
EXPLANATION :
As per the question, we are given vacuum tube. Vacuum tube can be of various types. Normally it contains two electrodes called cathode and anode which are enclosed in an evacuated glass chamber . There are also other types of vacuum tubes which contain extra electrodes like control grid .
The vacuum tube can be used as a rectifier. It means that it can be used as an electronic device which will convert alternating current into direct current. It may be a half wave rectifier or a full wave rectifier. Actually the direct current obtained during the rectification of alternating current is pulsating in nature.
Hence, the correct answer is that a vacuum tube can be used to change alternating current into direct current.