Meaning: 'Is charged up with' means that the device accumulates or stores heat energy, specifically from electricity generated by solar or wind sources.

Wrong options:

• substitutes for: This phrase implies replacement, which does not fit the context of storing heat; it suggests a different meaning altogether.
• is caught up with: This phrase suggests being involved in something, which does not apply to the context of storing heat energy.
• lives up to: This phrase means to meet expectations, which is irrelevant in the context of describing how the device functions.

Correct answer: which

Explanation: The word 'which' introduces a relative clause that provides additional information about the heat stored in the sand, making it grammatically correct and contextually relevant to the preceding clause.

Meaning: 'Which' is a pronoun used to refer to a specific thing mentioned previously, in this case, the heat that can warm homes.

Wrong options:

• whatever: This term is too vague and does not provide the necessary specificity required in this context.
• however: This word indicates contrast and does not fit the context of providing additional information about the heat.
• that: While 'that' could work, 'which' is more appropriate here for non-restrictive clauses, providing extra information.

Correct answer: while

Explanation: The word 'while' indicates a simultaneous action or contrast, effectively linking the quick addition of solar panels and wind turbines to the challenges they present, fitting the context of the sentence.

Meaning: 'While' means at the same time as or although, indicating that both actions are occurring together.

Wrong options:

• except: This word implies exclusion, which does not fit the context of discussing simultaneous developments in energy production.
• therefore: This term indicates a conclusion or result, which does not align with the context of contrasting actions.
• then: This word suggests a sequence of events rather than simultaneous actions, making it unsuitable here.

Correct answer: Right now

Explanation: The phrase 'Right now' indicates the current situation, emphasizing the immediacy of the issue regarding battery technology, which fits well with the overall context of the passage.

Meaning: 'Right now' means at this moment or currently, highlighting the present state of battery technology.

Wrong options:

• Of course: This phrase implies agreement or acknowledgment, which does not convey the urgency of the current situation.
• Besides: This word suggests additional information but lacks the immediacy needed in this context.
• Apart from: This phrase indicates separation or distinction, which does not fit the context of discussing the current state of battery technology.

Correct answer: as well as

Explanation: The phrase 'as well as' indicates addition, suggesting that the technology should be able to produce both electricity and heat, fitting grammatically and contextually into the sentence.

Meaning: 'As well as' means in addition to, indicating that the device should provide both forms of energy.

Wrong options:

• inside: This term implies being within something, which does not fit the context of energy production.
• despite: This word indicates contrast and does not align with the context of discussing capabilities of the technology.
• along: This term suggests movement or accompaniment, which does not fit the context of energy generation.