题目：A critical point of the waterfall software life-cycle model is that:
A. development thus ensuring buy-in from the clients.
B. risk analysis and risk resolution are explicitly defined for each stage.
C. no modifications can be made as each stage once signed off by the clients and developers becomes definitive.
D. no phase is complete until the documentation for that phase has been completed and the deliverables for that phase have been approved by the SQA group.
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