Diffusion rate decreases with distance in agar gel

Steps:

• Alkali solution diffuses from the well into the agar, reacting with the embedded indicator to form a pink color.
• The circular spread indicates radial diffusion from a point source.
• Time to reach markers increases progressively, showing slower spread over greater distances.
• This pattern follows Fick's first law, where flux decreases with distance squared in simple diffusion.

Why B is correct:

• Simple diffusion is passive movement down a concentration gradient without proteins; the increasing time matches the inverse square relationship in Fick's law (J = -D * ΔC / Δx).

Why the others are wrong:

• A: Facilitated diffusion involves carrier proteins for faster/specific transport, but the slowing rate shows no such acceleration.
• C: Indicator is stationary in agar; facilitation applies to moving substances, not the fixed indicator.
• D: Indicator does not diffuse; the pink color results from alkali diffusion triggering indicator reaction.

Final answer: B