C. Series
Consider resistors in a circuit - if all the resistors in the circuit are in series and one of the resistors fails then no current can flow thru the circuit,
If the resistors are in parallel then then each resistor experiences the same voltage drop regardless of whether or not any resistor in particular is carrying current.
The ball accelerates because of gravity.
Answer:
W = 0
Explanation:
We are given with, a construction worker is carrying a load of 40 kg over his head and is walking at a constant velocity. He travels a distance of 50 m.
The work done by an object is given by :

F = ma
So,

m is mass
a is acceleration
d is displacement
The worker is moving with constant velocity, its acceleration will be 0. So, the work done by the worker is 0.
These are known as balanced forces because they will not change the motion of the object, and it will remain at rest unless forces become unbalanced- meaning they would be unequal and not opposing.