For which optical devices does f have a negative value?

i think d

Explanation:because concave and convex mirrors have f negative value

B and D   ( c )

Explanation:

The optical devices that have f ( focal length ) as negative is the diverging lens ( image b ) and a convex mirror ( image d ).

An optical device is said to have a negative focal length when the image been focused or viewed on it appears on the same side as with the object been viewed as this is observed  when images are viewed through a diverging lens ( image b ) and also when an image is viewed through a convex mirror ( image d ). when an image is viewed through an optical device and the image appears on the opposite side the optical device is a said to have a positive focal length ( f )  

the negative f ( focal length ) means that the focal point of the optical device is on the same side of the lens as with where the object is placed

powerful computers now, not then

t = 1/400 h   ⇔ 0.0025 h ⇔ 9s ⇔ 0.15 m

any of the answers are correct.