Compare the partial pressures of oxygen and carbon dioxide in alveolar air versus atmospheric air.

Atmospheric air contains approximately 21% oxygen (O$_2$) and a very small percentage of carbon dioxide (CO$_2$), around 0.04%. At sea level, the partial pressure of O$_2$ (pO$_2$) in atmospheric air is about 159 mmHg, and the partial pressure of CO$_2$ (pCO$_2$) is about 0.3 mmHg.

Alveolar air is the air present in the alveoli of the lungs. When atmospheric air is inhaled, it mixes with the residual air already present in the lungs. More importantly, continuous gas exchange occurs in the alveoli:

• Oxygen (O$_2$) constantly diffuses from the alveoli into the blood. This process removes O$_2$ from the alveolar air.
• Carbon dioxide (CO$_2$) constantly diffuses from the blood into the alveoli. This process adds CO$_2$ to the alveolar air.

Due to the continuous gas exchange:

• The pO$_2$ in alveolar air will be lower than in atmospheric air because O$_2$ is constantly being absorbed into the blood. (Typical alveolar pO$_2$ is about 104 mmHg).
• The pCO$_2$ in alveolar air will be higher than in atmospheric air because CO$_2$ is constantly being released from the blood into the alveoli. (Typical alveolar pCO$_2$ is about 40 mmHg).

• (A) pO$_2$ lesser, pCO$_2$ higher: This matches our analysis.
• (B) pO$_2$ higher, pCO$_2$ lesser: Incorrect.
• (C) pO$_2$ higher, pCO$_2$ higher: Incorrect for pO$_2$.
• (D) pO$_2$ lesser, pCO$_2$ lesser: Incorrect for pCO$_2$.

The final answer is $\boxed{\text{A}}$