Meaning: 'But' is used to introduce a statement that contrasts with or seems to contradict something that has been said previously.

Wrong options:

• and: 'And' is a conjunction that adds information rather than contrasting it, making it unsuitable for this context.
• or: 'Or' suggests an alternative, which does not fit the contrasting nature of the sentence.
• almost: 'Almost' indicates a degree of approximation, which does not convey the necessary contrast between the snake's actual colour and its name.

Correct answer: fastest

Explanation: The superlative form 'fastest' is appropriate here as it indicates that the Black Mamba is one of the quickest snakes in the world. This aligns with the grammatical requirement for a superlative adjective to describe one among many.

Meaning: 'Fastest' means the highest speed among a group, indicating that this snake is among the quickest of all snakes.

Wrong options:

• fast: 'Fast' is a positive adjective and does not convey the superlative degree needed in this context.
• most fast: 'Most fast' is grammatically incorrect as 'fast' is already a one-syllable adjective that takes the superlative form 'fastest.'
• more fast: 'More fast' is also incorrect because 'fast' does not require 'more' to indicate a comparative degree.

Correct answer: which

Explanation: The word 'which' is a relative pronoun that introduces a non-defining clause, providing additional information about the venom of the Black Mamba. It fits grammatically and contextually as it connects the clause to the noun it describes.

Meaning: 'Which' is used to refer to a specific thing or group, in this case, the venom of the Black Mamba, providing further details about it.

Wrong options:

• when: 'When' is an adverb of time and does not relate to the subject of venom, making it contextually inappropriate.
• who: 'Who' is used for people, not for things like venom, thus it does not fit here.
• that: 'That' could be used, but 'which' is more appropriate in this context for non-defining clauses.

Correct answer: cause

Explanation: The verb 'cause' is correctly used here to indicate that the venom leads to damage, fitting grammatically as it aligns with the subject 'toxins' and the context of the sentence.

Meaning: 'Cause' means to make something happen, in this case, the damage to the victim's nervous system and heart due to the venom.

Wrong options:

• make: 'Make' is too vague and does not specifically convey the idea of leading to damage, making it less precise in this context.
• avoid: 'Avoid' implies preventing something, which is the opposite of what the sentence intends to convey about the effects of the venom.
• force: 'Force' suggests coercion or compulsion, which does not accurately describe the action of the venom.

Correct answer: rate

Explanation: The noun 'rate' is appropriate here as it refers to the frequency or proportion of mortality, fitting grammatically as it aligns with the context of discussing the likelihood of death from the venom.

Meaning: 'Rate' refers to a measure, quantity, or frequency, particularly in relation to mortality in this context.

Wrong options:

• rating: 'Rating' implies a score or evaluation, which does not fit the context of discussing mortality rates.
• rates: 'Rates' is a plural form that does not match the singular context of 'the mortality rate' being discussed.
• rated: 'Rated' is a past participle that does not function as a noun in this context, making it grammatically incorrect.