Occasionally open the calendar solar terms.

As can be seen from the schematic diagram of the earth revolution in Figure 2, when the earth is in position A, the sun shines directly on the Tropic of Cancer, which is the summer solstice in the northern hemisphere, and the date is around June 22nd. When the earth is in position B, the latitude of direct sunlight is the equator, which is the vernal equinox in the northern hemisphere, and the date is around March 2 1; When the earth is in position D, the latitude of direct sunlight is the equator, which is the autumn equinox in the northern hemisphere, and the date is around September 23rd. When the earth is in position C, the latitude of direct sunlight is tropic of Capricorn, which is the winter solstice in the northern hemisphere, and the date is about 65438+February 22nd.

The solar term in figure 1 is the vernal equinox, which is the position b in figure 2.

Therefore, choose: B.