Science grade 8 does anyone know this answer to this science question? select the sentence that is true about how a person changes the pitch of sounds when singing. a person stretches the vocal cords to produce higher-frequency sound waves. a person stretches the vocal cords to produce lower-frequency sound waves. a person relaxes the vocal cords to produce higher-frequency sound waves. a person relaxes the vocal cords to produce lower-frequency sound waves.

)benzene and toluene form nearly ideal solutions. consider an equimolar solution of benzene and toluene. at 20 °c the vapour pressures of pure benzene and toluene are 9.9 kpa and 2.9 kpa, respectively. the solution is boiled by reducing the external pressure below the vapour pressure. calculate (i) the pressure when boiling begins, (ii) the composition of each component in the vapour, and (iii) the vapour pressure when only a few drops of liquid remain. assume that the rate of vaporization is low enough for the temperature to remain constant at 20 °c.