How to give the spheres exactly equal but opposite charges? A. Make sure spheres are not in contact with each other. Rub the glass rod on the silk to charge the rod positively (hence to charge piece of silk negatively). Bring the rod near, but not touching, one of the spheres. Do the same with piece of silk and another sphere. Remove the galss rod and the piece of silk.
B. Make sure spheres are not in contact with each other. Rub the glass rod on the silk to charge the rod positively (hence to charge piece of silk negatively). Touch one of the spheres with the glass rod then touch another one with the piece of silk.
C. Put the two metal spheres in contact with each other. Then rub the glass rod on the silk to charge the rod positively (hence to charge piece of silk negatively). Touch one of the spheres with the glass rod then separate the spheres. Touch another sphere with the piece of silk.
D. Put the two metal spheres in contact with each other. Then rub the glass rod on the silk to charge the rod positively (hence to charge piece of silk negatively). Bring the rod near, but not touching, one of the spheres. Separate the spheres and remove the rod.

B) Make sure spheres are not in contact with each other. Rub the glass rod on the silk to charge the rod positively (hence to charge piece of silk negatively). Touch one of the spheres with the glass rod then touch another one with the piece of silk.

Explanation:

In static electricity, frictional rubbing can excite electrons, and make them jump from one material to another. Rubbing the glass rod and the silk together makes electron flow from the glass rod to the silk piece. On separation, the rod and silk will be seen to have equal amount of charge but, the glass rod will be positively charged from losing electron, and the silk piece will be negatively charged from gaining extra electrons. When they are each made to touch the metal spheres that are not in contact, the spheres will acquire equal amount of charges opposite the charge on the material that was used to charge them.

i don't know that i agree with that but what's the question ?

there is no correct list among the choices you've provided.

the list might look something like this:

-- baby human

-- piccolo

-- flute

-- soprano singer

-- trumpet

-- tenor singer

-- trombone

-- baritone singer

-- baritone horn

-- bass singer

-- tuba

since i had to make up the list myself, i made it easy on myself, and i didn't try to shuffle the string instruments and woodwinds into it too.