Military personnel also use periscopes in some gun turrets and in armoured vehicles. More complex periscopes using prisms or advanced fibre optics instead of mirrors and providing magnification operate on submarines and in various fields of science
C is true, and just one of those has as much mass as about 1,840 electrons.
Answer: R = 394.36ohm
Explanation: In a LR circuit, voltage for a resistor in function of time is given by:

ε is emf
L is indutance of inductor
R is resistance of resistor
After 4s, emf = 0.8*19, so:





R = 394.36
In this LR circuit, the resistance of the resistor is 394.36ohms.
See this suggested solution.
1. Let a force F' is the vector sum of the forces P and Q, then it is shown on the attached picture and marked with red color.
2. according to the condition the force F holds the object, then F should have the same length as the force F' and the opposite direction.
3. using the conditions described in 2. the answer is C.