The Logic: Predictability vs. Chaos
The sentence describes a teacher whose behavior makes students "uneasy" because they "never knew what would please him."
- The Clue: The word "mercurial" means subject to sudden or unpredictable changes in mood.
- The Second Clue: The phrase "never knew what would please him" implies that the grading criteria were not fixed, logical, or consistent.
- The Goal: We need two synonyms that describe an approach that is unpredictable, impulsive, or based on random fancy rather than a set of rules.

The Correct Answers:
D. ambivalent E. whimsical
- Why they work: * Whimsical means acting or behaving in a capricious or impulsive manner. If a teacher's grading is whimsical, it changes based on a whim, which explains why students "never knew" what to expect.
- Ambivalent means having mixed feelings or contradictory ideas about something. In this context, an ambivalent approach to grading suggests the teacher isn't committed to a single standard, leading to the same unpredictability as a "mercurial" mood. Together, they characterize the "uneasy" lack of a clear standard.

Why the others fail:
- B. authoritarian and C. strict: These are "distractor traps." While a strict or authoritarian teacher might make students uneasy, their grading is usually very predictable (follow the rules or fail). The sentence specifically says the students "never knew" what to expect, which contradicts a strict, rule-based system.
- A. tardy: This means late. A teacher being late to grade doesn't explain why students wouldn't know how to please him.
- F. hidebound: This means narrow-minded and rigid in opinions. Similar to "strict," a hidebound teacher would be very consistent and predictable, not mercurial.

Final Completed Sentence
The teacher's mercurial mood changes and ambivalent / whimsical approach to grading made the students uneasy; they never knew what would please him or what would earn good marks.