Which two of the following changes (i-iv) usually tend to occur in the plain dwellers when they move to high altitudes (3500 m or more )
(i) Increase in red blood cell size
(ii) Increase in red blood cell production
(iii) Increased breathing rate
(iv) Increase in thrombocyte count

Options

A. (a) i and ii
B. (b) ii and iii
C. (c) iii and iv
D. (d) i and iv

Answer

To solve the question regarding the changes that occur in plain dwellers when they move to high altitudes (3500 m or more), we will analyze each of the provided statements (i-iv) and determine which two are correct.

### Step-by-Step Solution:

1. **Understanding High Altitude Effects**:
- When individuals from lower altitudes (plains) ascend to high altitudes, they encounter lower atmospheric pressure and reduced oxygen availability. This can lead to altitude sickness, characterized by symptoms like headache, nausea, and shortness of breath.

2. **Analyzing Each Statement**:
- **(i) Increase in red blood cell size**:
- This statement is incorrect. When acclimatizing to high altitudes, the body does not increase the size of red blood cells; rather, it increases the number of red blood cells.

- **(ii) Increase in red blood cell production**:
- This statement is correct. To compensate for the lower oxygen levels, the body increases the production of red blood cells, which helps in transporting more oxygen.

- **(iii) Increased breathing rate**:
- This statement is also correct. As the body tries to take in more oxygen, the breathing rate increases to enhance oxygen intake.

- **(iv) Increase in thrombocyte count**:
- This statement is incorrect. Thrombocytes (platelets) are involved in blood clotting, and there is no physiological need to increase their count in response to high altitude.

3. **Conclusion**:
- The two correct changes that usually occur in plain dwellers when they move to high altitudes are:
- (ii) Increase in red blood cell production
- (iii) Increased breathing rate

4. **Final Answer**:
- The correct options are (ii) and (iii). Therefore, the answer is option B, which includes these two statements.