Answer:
B. Smaller Particles
Step-by-step explanation:
We know that the rate of reaction increases with increase in the surface area of reactants.
The reacting particles in the log of wood are embedded inside the wood making only very minimal amount of particles available for reaction. This ultimately slows down the rate of combustion.
However, in saw dust, there is a greater availability of surface area of reactants hence the sawdust burns faster than logs.