Speed of wave is given as

Wavelength of the wave is given as

now from the formula of wave time period we can say




so it will have time period of T = 4 s
b is the answer i hope this helped...
Because of the magnets are actually electromagnetics aka what causes them to repel each other the atoms and the electrons will make a force of them pushing away from each other because the two magnetic poles are not north and south
All these resistors are in series so we can take the sum of them by:
Rtotal = R1 + R2 + R3......
So...
Rtotal = 2 + 3 + 4 + 6
Rtotal = 15
So now the total resistance in the circuit is 15 ohms and the potential difference applied to the circuit is 45 volts
Now we can use:
V = IR
Isolate for I
V/R = I
45/15 = I
I = 3 amps (A)