When does a gimbal lock occur on a three axis platform?$

• A. Inner gimbal approaches 90¬¨¬®‚Äö√†√ª
• B. Inner gimbal approaches 0¬¨¬®‚Äö√†√ª
• C. Outer gimbal approaches 90¬¨¬®‚Äö√†√ª
• D. Middle gimbal approaches 90¬¨¬®‚Äö√†√ª