风险A的EMV=50000*90%=45000;风险B的EMV=300000*30%=90000;机会A的EVM=100000*50%=50000;机会B的EMV=125000*15%=18750;风险B造成的损失是最大的，超过机会A和机会B,所以应该要优先考虑对该风险的应对，而不是先去把控机会。

To determine which risk or opportunity the project manager should consider first using expected monetary value (EMV) analysis, we calculate the EMV for each threat and opportunity. The EMV is calculated by multiplying the probability of each event by its potential impact (cost or savings).

1. **Threat A:**
- Probability: 90% or 0.9
- Impact: Cost of US$50,000
- EMV = 0.9 * (-50,000) = -$45,000

2. **Threat B:**
- Probability: 30% or 0.3
- Impact: Cost of US$300,000
- EMV = 0.3 * (-300,000) = -$90,000

3. **Opportunity A:**
- Probability: 50% or 0.5
- Impact: Savings of US$100,000
- EMV = 0.5 * 100,000 = $50,000

4. **Opportunity B:**
- Probability: 15% or 0.15
- Impact: Savings of US$125,000
- EMV = 0.15 * 125,000 = $18,750

Based on the EMV calculations, the project manager should consider:

- **Opportunity A** first, as it has the highest positive EMV of $50,000.

Opportunity A provides the greatest potential benefit to the project when considering both likelihood and impact. Therefore, the correct answer is **D: Opportunity A 机会A**.