Step 1: Context:
In 'Keeping Quiet', Pablo Neruda urges humanity to pause its restless activity for a moment of stillness and introspection.
Step 2: a) What we lose:
In our single-minded obsession with keeping life constantly busy and moving, we lose the opportunity for stillness, self-reflection, and genuine understanding of ourselves and our actions.
Step 3: b) The 'sadness':
The 'sadness' refers to humanity's tragic failure to ever truly understand itself — we remain trapped in constant activity, never pausing to reflect, which leaves us emotionally and spiritually disconnected from our own inner lives.
Step 4: c) How we threaten ourselves:
We threaten ourselves with death through our relentless, aggressive activity — symbolised elsewhere in the poem by wars, machines, and exploitation of nature — which pushes humanity toward self-destruction rather than harmony.
Final Answer:
a) The chance for stillness and self-understanding. b) The sadness of never understanding ourselves. c) Through our restless, destructive activity (wars, exploitation) that pushes us towards our own destruction.