A client reports substernal chest pain. Test results show electrocardiograph changes and an elevated cardiac troponin level. Which of the following should be the focus of nursing care（）
A. Improving myocardial oxygenation and reducing cardiac workload.
B. Confirming a suspected diagnosis and preventing complications.
C. Reducing anxiety and relieving pain.
D. Eliminating stressors and providing a nondemanding environment.
