The main reason(s) for warm summers in northern middle latitudes is that:

The correct answer is: E. Only (b) and (c) are correct.

The Earth is closest to the sun in January, not in summer. However, the Earth’s axis is tilted, so the Northern Hemisphere is tilted towards the sun in summer and away from the sun in winter. This means that the sun is higher in the sky in summer and we receive more direct solar radiation. This is the main reason for warm summers in the Northern Hemisphere. The longer days in summer also contribute to warmer temperatures, but to a lesser extent.

Here is a more detailed explanation of each option:

• A. The earth is closer to the sun in summer. This is not correct. The Earth is closest to the sun in January, not in summer.
• B. The sun is higher in the sky and we receive more direct solar radiation. This is correct. The Earth’s axis is tilted, so the Northern Hemisphere is tilted towards the sun in summer and away from the sun in winter. This means that the sun is higher in the sky in summer and we receive more direct solar radiation. This is the main reason for warm summers in the Northern Hemisphere.
• C. The days are longer in summer. This is also correct. The days are longer in summer because the Earth’s axis is tilted. This means that the Northern Hemisphere receives more sunlight in summer than in winter. However, the longer days are not the main reason for warm summers. The main reason is that the sun is higher in the sky in summer and we receive more direct solar radiation.
• D. All of the above. This is not correct. Only options (b) and (c) are correct.
• E. Only (b) and (c) are correct. This is the correct answer.