Over which key should your thumbs hover when not in use?
a. alt
b. ctrl
c. enter
d. space

D. Space

Explanation:

-The Alt key is used to modify the function of another key that is pressed and these keys are located on both side of the space bar.

-The Ctrl key is used to perform a specific function when pressed with another key and these keys are located in the bottom left and right of the keyboard.

-The enter key is used to select an option or take the cursor to the next line and it is located on the right side of the keyboard.

-Space key is used to enter a space between words and it is a horizontal bar located in the lower part of the keyboard.

According to this and considering that when using the keyboard and your thumbs are not in use, they are straight and the rest of your fingers curve over the keys, they hover over the lower part of the keyboard where the space key is located. Because of that, the answer is that your thumbs hover when not in use over the space key.