54:16 Which statement describes a property of covalent compounds?
They have a high boiling point because of strong intermolecular forces
They have a low boiling point because of weak intermolecular forces
They have a low melting point because of strong intermolecular forces
They have a high melting point because of weak intermolecular forces
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acid rain does not usually kill trees directly. instead, it is more likely to weaken the trees by damaging their leaves, limiting the nutrients available to them, or poisoning them with toxic substances slowly released from the soil. the main atmospheric pollutants that affect trees are nitrates and sulphates.

under a microscope, cancer cells may look very different from normal cells. they often have different sizes and some may be larger than normal while others are smaller. cancer cells are often abnormally shaped and the control centre of the cell (the nucleus) may have an abnormal appearance.

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