How can an element be identified using its emission spectrum? a. by measuring the frequencies of the spectral lines, and calculating the energies that these correspond to b. by measuring the wavelengths of the spectral lines, and then calculating the differences between adjacent wavelengths c. by heating the unknown element, and then comparing the emitted spectrum with known spectra of the elements d. by identifying the color of the first line of the spectrum, and comparing this to a chart of known spectra

The correct choice is C .

Technically, choice-A is the process involved.  But it's already been done,
and widely published, and the spectrum of every element is easily available. 

For you to take the time and go to the trouble of constructing the spectra
would be like inventing wheels if you want to ride a bicycle. 

Is a  vector quantity  that refers to "the rate at which an object changes its position." imagine a person moving rapidly - one step forward and one step back - always returning to the original starting position. while this might result in a frenzy of activity, it would result in a zero velocity. because the person always returns to the original position, the motion would never result in a change in position. since velocity is defined as the rate at which the position changes, this motion results in zero velocity. if a person in motion wishes to maximize their velocity, then that person must make every effort to maximize the amount that they are displaced from their original position. every step must go into moving that person further from where he or she started. for certain, the person should never change directions and begin to return to the starting position.