How do scientists infer the presence of black holes since they can't be directly observed?

Complete the statement about memory improvement. ( )involves organizing and breaking down information into easier groups to expand capacity. rehearsal is the verbal repetition of information. these techniques are especially important for the improvement of ( ) memory.

An ionic compound can conduct electricity when a it is dissolved in water b its crystals are ground into powder c it is in solid form d it is heated to just below its melting point

Acertain radio station broadcasts radio waves that have a wavelength of 298cm what is its broadcast frequency?