Consider the following sentences:
(i) The number of candidates who appear for the IELTS examination is staggering.
(ii) A number of candidates from my class are appearing for the IELTS examination.
(iii) The number of candidates who appear for the IELTS examination are staggering.
iv) A number of candidates from my class is appearing for the IELTS examination.
Which of the above sentences are grammatically correct?