Which best blocks x-ray particles and can therefore be observed more closely through the use of this technology?

X-rays are high energy electrons that can cause damage when exposed under extreme conditions. The best technology that can block it is t using a lightweight type of metal foam. It can take in high energy collisions which also exhibits high forces. it does not only block x-rays but also, neutron radiation and gamma rays.

one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.