Meaning: 'Restrict' means to limit or control something, in this case, the flow of water in a river.

Wrong options:

• channel: The word 'channel' refers to directing water through a specific path, which does not convey the idea of limiting or controlling the flow as effectively as 'restrict'.
• cross: The word 'cross' implies moving from one side to another, which does not relate to the control of water flow in the context of dam construction.
• span: The word 'span' suggests extending across a distance, which does not fit the intended meaning of controlling water flow.

Correct answer: Throughout

Explanation: The word 'Throughout' is appropriate as it indicates the entire duration of history, fitting grammatically and contextually to describe the continuous use of dams over time.

Meaning: 'Throughout' means in every part or during the whole time, emphasizing the long-standing use of dams in history.

Wrong options:

• In: The word 'In' suggests a specific point in time, which does not convey the ongoing nature of dam usage across history.
• For: The word 'For' implies purpose or duration but does not indicate the comprehensive scope of time as 'Throughout' does.
• Against: The word 'Against' suggests opposition, which does not fit the context of discussing the historical use of dams.

Correct answer: however

Explanation: The word 'however' serves as a discourse marker that introduces a contrast, fitting well grammatically and contextually to highlight the negative effects of damming rivers after discussing their benefits.

Meaning: 'However' means 'but' or 'on the other hand', indicating a shift in focus to the negative aspects of dams.

Wrong options:

• again: The word 'again' implies repetition, which does not fit the context of introducing a contrasting idea about negative effects.
• never: The word 'never' suggests a lack of occurrence, which contradicts the context of acknowledging negative effects.
• also: The word 'also' indicates addition, which does not appropriately signal a contrast as needed in this context.

Correct answer: knocked down

Explanation: The phrase 'knocked down' is a colloquial expression meaning to demolish or destroy, fitting grammatically and contextually to describe the destruction of homes for dam construction.

Meaning: 'Knocked down' means to demolish or tear down, referring to the destruction of homes to make way for the dam.

Wrong options:

• traded up: The phrase 'traded up' implies exchanging for something better, which does not fit the context of destruction of homes.
• worn off: The phrase 'worn off' suggests fading or diminishing, which does not relate to the act of demolishing homes.
• moved on: The phrase 'moved on' implies leaving a place or situation, which does not convey the idea of destruction as effectively as 'knocked down'.