The graphic below shows the propagation of S-waves from an earthquake. The arrows represent the S-waves,...

Question 20
The graphic below shows the propagation of S-waves from an earthquake. The arrows represent the S-waves, and the numbers represent two different types of Earth materials.
Which best explains why the S-waves do not travel through material #2?
A. because material #2 is a solid
B. because material #2 is a liquid
C. because material #2 is cooler than material #1
D. because material #2 is hotter than material #1
Question 21
The combustion of propane is represented below. For the reaction to occur, the energy of the system must meet the activation energy threshold.
C₃H₈ (g) + 5 O₂ (g) → 3 CO₂ (g) + 4 H₂O (g)
A. because the pressure on the system decreases
B. because the reactants become more flammable
C. because carbon dioxide (CO₂) traps the additional heat
D. because more molecules collide with greater force and frequency