The equinox

a. is when the subsolar point is at one of the tropics.
b. is the longest day of the year at any given place.
c. occurs four times during the year.
d. has 12 hours of day and 12 hours of night for all locations.

The equinoxes (from Latin aequinoctium (aequus nocte), "equal night") are the times of the year when the Sun is located in the plane of the celestial equator and comes from the Latin aequinoctium (aequus nocte), "equal night". It occurs twice a year: between March 19 and 21 and between September 21 and 24 of each year. As the name implies, on the dates on which the equinoxes occur, the day lasts approximately equal to that of the night in all places on Earth.

The equinoxes are used to set the beginning of spring and autumn in each hemisphere.

Therefore the correct answer is D.

it is given that trillions of microbes make up to 1% to 3% of the body mass.

let's take average of this percentage: 2%

consider the average mass of the human body as 70 kg.

then, trillions of microbes would have mass  

therefore, 100 trillion micro-organisms would weigh:

this is an approximate value and could change depending on the mass of the human body and percentage of the microbes present in it.

average mass per microorganism =