Among the 24 solar terms, the sun shines vertically on the tropic of Capricorn only during the winter solstice. At noon from winter to Sunday, it is at the horizontal line in Beijing.

40+23.5 = 63.5, that is, the included angle between sunlight and the plane of Beijing urban area is 63.5.

In a right-angled triangle with an acute angle of 63.5, the ratio of the opposite side of 63.5 to its adjacent right-angled side is about 2: 1.

Therefore, the shadow length of a straight bar with a length of 1m should be 2m.

So choose C.