Da VIP Academy investigation summary

Students grew 40 bean plants under four light conditions for 21 days. Every plant got the same soil, pot size, and 60 mL of water per day.

Group  Light per day  Average height (cm)
A      2 hours        6.1
B      6 hours        13.4
C      12 hours       19.8
D      18 hours       20.2

A student repeats the study but also gives Group D twice as much water. Why does this weaken the investigation?

• A.It makes the study take longer.
• B.Two variables change at once, so height differences cannot be attributed to light alone.
• C.Water does not affect plants.
• D.It increases the sample size too much.