To determine the correct observation that should occur in the well with the missing data point, we need to analyze the existing data and use pattern recognition.
Here's the given table with observations filled in where possible:
[tex]\[
\begin{array}{|c|c|c|c|c|}
\hline
& A & B & C & D \\
\hline
A & X & \text{clear/colorless to clear/pink} & \text{warm to touch} & \text{none} \\
\hline
B & \text{clear/colorless to clear/pink} & X & \text{none} & \text{cool to touch} \\
\hline
C & \text{warm to touch} & \text{none} & X & [?] \\
\hline
D & \text{none} & \text{cool to touch} & \text{bubbles} & X \\
\hline
\end{array}
\][/tex]
To find the missing observation for the combination of C and D, we take the following steps:
1. Review Existing Observations:
- C paired with A: warm to touch
- C paired with B: none
- C paired with D: [?]
- D paired with A: none
- D paired with B: cool to touch
- D paired with C: bubbles
2. Analyze Patterns:
- C paired with A and C paired with D both produced temperature changes (warm, bubbles).
- D paired with B produced a temperature change (cool to touch).
3. Identify Consistency:
- The observed combinations show that any pair involving compound D results in a noticeable and often significant effect (none, cool to touch, bubbles).
- Compound C, when combined with D, likely results in a pronounced reaction similar to bubbles, as seen in compound D’s other pairings that produce significant visual or thermal changes.
Based on this pattern, the missing observation when compounds C and D are mixed is most logically consistent with producing bubbles, much like D does when combined with other compounds.
Thus, the correct choice is:
A. bubbles
