When we have separated them, using nothing more than beginner's instinct (disqualifying ones that look too real to be true, for example), Roxanne smiles.

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"What we have today cannot be separated from alternative energy, " Zhang told journalists visiting his home last year.

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"We have different business plans for different airlines, " he says, explaining how tomorrow's planes could be separated into zones.

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"If we find the parachute we will know that the lander is no more than a kilometre away because if everything went right the two would have separated one kilometre above the surface and there wouldn't be much distance between them when they came down, " he said.

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