Vocabulary in context: in the passage about the power of thought, choose the noun that best completes the clause "for it is the ______ of the other two." referring to thought as related to word and deed.

Introduction / Context:
The passage explains that among thought, word and deed, thought is special because it comes first. The sentence with the blank reads, "Of the three activities of thought, word and deed, perhaps thought is the most potent and powerful force, for it is the ______ of the other two." The noun in the blank must describe thought's role as that which comes before and gives rise to words and deeds.

Given Data / Assumptions:
Thought happens first in the mind; words and deeds follow from it.
Later in the passage, it is said that success is achieved twice, first in the mind and second in the real world.
Options are sponsor, precursor, sensor, spoiler and an added distractor product.
The context is about causation and order, not about funding or cancelling.

Concept / Approach:
Precursor means something that comes before another and indicates or leads to it. In this context, thought is the precursor of word and deed because it precedes them and causes them. Sponsor is someone who supports or funds something, sensor is a device that detects input, spoiler is something that ruins enjoyment, and product would describe something that comes after a process. Only precursor accurately captures the intended relationship.

Step-by-Step Solution:
Step 1: Focus on the idea that thought must come before word and deed, because we think before we speak and act. Step 2: Identify which option means something that comes before another in time and serves as its forerunner. That definition matches precursor. Step 3: Test precursor: thought is the precursor of the other two. This means thought is the earlier stage that leads to words and deeds, which fits the passage. Step 4: Reject sponsor, which implies financial or moral backing but not necessary temporal priority in this sense. Step 5: Reject sensor, spoiler and product because none of them fits the logical relationship being described.

Verification / Alternative check:
When we use precursor, the sentence clearly expresses that thought is the original stage from which both speech and action originate. This corresponds well with the later statement about success occurring first in the mind. No other option produces a comparably coherent picture. Thought cannot be the product of word and deed if we accept the passage's explanation; it must be the source, not the result.

Why Other Options Are Wrong:
Sponsor suggests someone or something that supports another entity, which does not match the idea that thought temporally precedes words and deeds as an inner stage. Sensor belongs to the domain of electronics or biology and would imply detecting stimuli rather than originating actions. Spoiler implies something that ruins an outcome and has no logical link here. Product would invert the cause effect relation by suggesting that thought comes after words and deeds, which contradicts the explanation that success is achieved first in thought.

Common Pitfalls:
Students sometimes confuse precursor and sponsor because both can be associated with events that come before another, but only precursor directly emphasises chronological succession and causation. Carefully checking dictionary meanings or recalling common collocations such as precursor to change or precursor of a new movement will help in avoiding mistakes.

Final Answer:
The noun that best completes the sentence is precursor, so option precursor is correct.