Milk Enables Continued Growth in Rats

Steps:

• Examine the graph: the no-milk group plateaus after day 18, showing no further mass gain.
• Compare to the milk group: mass continues increasing after day 18 with 3 cm³ milk daily.
• Note controlled conditions: differences result solely from milk presence.
• Conclude milk supplies growth-essential nutrients, as its absence halts development.

Why D is correct:

• The controlled experiment isolates milk's role; continued mass gain with milk versus stagnation without proves it provides required carbohydrates, proteins, fats, minerals, and vitamins for growth.

Why the others are wrong:

• A: Masses differ on day 46; milk group is heavier due to ongoing growth.
• B: Milk is vital, as its removal stops mass increase.
• C: No acceleration; one group merely maintains prior growth rate, the other halts.

Final answer: D