Which of the following IS NOT true about the roles of chemical synapses and their neurotransmitters? Some bind with the postsynaptic cell and interrupt its firing, which is called an inhibitory response.

Some bind with the postsynaptic cell and cause the firing this second cell, which is called an excitatory response.

Some are only electrical, so no neurotransmitters are necessary in these chemical synapses.

One hundred or more types of neurotransmitters have been identified, and they have different excitatory and inhibitory effects.

Uranium has two naturally occurring isotopes. 238u has a natural abundance of 99.3% and 235u has an abundance of 0.7%. it is the rarer 235u that is needed for nuclear reactors. the isotopes are separated by forming uranium hexafluoride uf6, which is a gas, then allowing it to diffuse through a series of porous membranes. 235uf6 has a slightly larger rms speed than 238uf6 and diffuses slightly faster. many repetitions of this procedure gradually separate the two isotopes. what is the ratio of the rms speed of 235uf6 to that of 238uf6? express your answer to five significant figures.