Photoelectrons are emitted by a metal surface only when the light shining on the surface exceeds a certain minimum:
a wavelength
b frequency
c velocity
d charge

Explanation:

Photoelectric effect is described as the phenomenon of emission of electrons when light falls on the metal surface.

The theory assumes particle theory of light.

When photons hit the metal surface,the energy of photon is used to overcome to binding force between electron and nucleus.

The energy of photon is  given by where is placks constant and is frequency.

When the frequency of photon is low,the energy of photon may not be sufficient to emit a electron.

So,emission requires a minimum frequency.

car a and b are moving in opposite directions

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cell differentiation  is how generic embryonic  cells  become specialized  cells. this occurs  through a process called gene expression. gene expression is the specific combination of genes that are turned on or off (expressed or repressed), and this is what dictates how a  cell  functions.

explanation: