The marking scheme for the BPSC Civil Services preliminary exam is as follows:

1. Exam Pattern:
- The preliminary exam consists of two papers: General Studies Paper I and General Studies Paper II (CSAT).
- Both papers are objective type (multiple choice questions) and are conducted in offline mode.
- Each paper carries 150 marks, making the total marks for the preliminary exam 300.

2. Marking Criteria:
- For every correct answer, candidates are awarded 1 mark.
- There is no negative marking for incorrect answers.

3. General Studies Paper I:
- This paper tests candidates on various subjects such as General Science, Indian History, Indian Polity, Geography, Economy, etc.
- It consists of 100 questions, each carrying 1 mark.
- The total marks for this paper are 100.

4. General Studies Paper II (CSAT):
- This paper is commonly known as the Civil Services Aptitude Test (CSAT).
- It assesses candidates' comprehension, logical reasoning, analytical ability, decision making, problem-solving, and general mental ability.
- It consists of 80 questions, each carrying 1.5 marks.
- The total marks for this paper are 200.

5. Qualifying Criteria:
- In order to qualify for the main exam, candidates need to score the minimum qualifying marks in the preliminary exam.
- The minimum qualifying marks are determined by the Bihar Public Service Commission (BPSC) and may vary from year to year.
- Candidates who score above the minimum qualifying marks are eligible to appear for the main exam.

In conclusion, the marking scheme for the BPSC Civil Services preliminary exam follows a simple pattern. Candidates are awarded 1 mark for each correct answer, and there is no negative marking for incorrect answers. The exam consists of two papers, General Studies Paper I and General Studies Paper II (CSAT), with a total of 300 marks. The minimum qualifying marks are determined by the BPSC and candidates who score above the qualifying marks are eligible for the main exam.