Based on the data you provided, the relationship between altitude and song frequency can be analyzed as follows:
- At low altitude (below 1500 Hz), 80 birds were observed singing.
- At medium altitude (below 1500 Hz), 50 birds were observed singing.
- At high altitude (below 1500 Hz), only 20 birds were observed singing.
From this information, it is evident that as the altitude increases, the number of birds singing at frequencies below 1500 Hz decreases. This suggests that higher altitudes are associated with lower song frequencies.
Therefore, the correct answer is:
C. Higher altitudes are correlated with lower song frequencies.