**Core Concept**
In civil cases, conduct money is a form of compensation awarded to witnesses for their expenses, time, and inconvenience incurred while attending court. This concept is rooted in the principle of witness protection and support, ensuring that individuals who contribute to the legal process are fairly compensated.

**Why the Correct Answer is Right**
The correct answer is that conduct money is decided by the judge. The judge has the authority to determine the amount of compensation based on the witness's expenses, travel time, and other relevant factors. This decision is typically made during the court proceedings, and the judge may consider various factors, including the witness's testimony, the complexity of the case, and the distance traveled to attend court. The judge's decision is final and binding, ensuring that witnesses are fairly compensated for their time and expenses.

**Why Each Wrong Option is Incorrect**
* **Option A:** Opposite pay refers to the reimbursement of expenses by the opposing party, but it is not the decision-maker in determining conduct money.
* **Option C:** While doctors may be called as expert witnesses, they do not have the authority to decide conduct money.
* **Option D:** Witnesses themselves do not determine their own conduct money, as this is a decision made by the judge.

**Clinical Pearl / High-Yield Fact**
It's essential for medical professionals to be aware of the rules and regulations governing conduct money, as they may be called as witnesses in court proceedings. Understanding the process of determining conduct money can help them navigate the legal system more effectively.

**✓ Correct Answer: B. Judge. The judge decides conduct money in civil cases.**