Answer:
B. 6 cm
Explanation:
First, we calculate the spring constant of a single spring:

where,
k = spring constant of single spring = ?
F = Force Applied = 10 N
Δx = extension = 4 cm = 0.04 m
Therefore,

Now, the equivalent resistance of two springs connected in parallel, as shown in the diagram, will be:

For a load of 30 N, applying Hooke's Law:

Hence, the correct option is:
<u>B. 6 cm</u>
There is still air inside of a house, which is pushing the roof upwards, so the forces are equal and the roof is not crushed.
Answer:
He can throw it away from himself.
Explanation:
Newtons Third Law says that everything has an equal, yet opposite reaction on other objects.