Meaning: 'Should' is used to express obligation or duty, implying that it is advisable for farmers to fertilize their fields.

Wrong options:

• must: 'Must' implies a strong obligation, which may not be appropriate in this context where advice is more suitable.
• need: 'Need' suggests necessity but lacks the advisory tone that 'should' conveys, making it less fitting here.
• can: 'Can' indicates ability or possibility, which does not convey the recommendation that is intended in this context.

Correct answer: has

Explanation: The word 'has' is the correct form of the verb to agree with the singular noun 'fertilizer,' fitting grammatically and contextually as it indicates that fertilizer possesses a strong effect.

Meaning: 'Has' means to possess or own, indicating that fertilizer possesses a strong effect on productivity.

Wrong options:

• had: 'Had' is the past tense form, which does not fit the present context of discussing fertilizer's current effects.
• have: 'Have' is plural and does not agree with the singular subject 'fertilizer,' making it grammatically incorrect here.
• having: 'Having' is a gerund and does not function as a main verb in this context, thus it does not fit grammatically.

Correct answer: problem

Explanation: The word 'problem' fits contextually as it refers to an issue that needs addressing, aligning with the overall discussion about farmers' knowledge of fertilizer usage.

Meaning: 'Problem' refers to a matter or situation regarded as unwelcome or harmful and needing to be dealt with.

Wrong options:

• question: 'Question' implies an inquiry rather than an issue, which does not align with the context of discussing farmers' challenges.
• conclusion: 'Conclusion' suggests a final decision or judgment, which is not relevant to the context of identifying an issue.
• answer: 'Answer' implies a solution or response, which does not fit the context of identifying a challenge faced by farmers.

Correct answer: how much

Explanation: The phrase 'how much' is appropriate as it refers to the quantity of fertilizer, fitting grammatically and contextually in the sentence about farmers' uncertainty.

Meaning: 'How much' is used to inquire about the quantity of something, in this case, fertilizer.

Wrong options:

• how many: 'How many' is used for countable nouns, while 'fertilizer' is uncountable in this context, making it incorrect.
• however: 'However' is a transitional word that introduces contrast, which does not fit the context of asking about quantity.
• so much: 'So much' implies a large quantity but does not function as a question, making it unsuitable in this context.