Cube B has a lower heat capacity. Cube B cools faster. A thermometer is laid out in direct sunlight. The thermometer always measures the temperature of the thermometer. But you can still put your hand in the oven as long as you don't touch anything.
Air is a very poor thermal conductor.
Therefore, its heat cannot be effectively transferred to your hand. If you touch the shelves made of iron, heat can be transferred quickly and you will be burned. If you leave your hand in the oven for too long, however, even just the air will transfer enough thermal energy to roast you. The body looses heat due to breathing. Cold air is inhaled and warm air exhaled.