In option A singular form of noun i.e question is used which is wrong.
In option B plural form of noun is used which is correct.
In option C a great many is always followed by plural noun and pluralverb.
In option D again plural form of investigation is used which is wrong as in sentence we are talking about only one investigation.
Where are leukocytes, or white blood cells, produced in the body?
Which type of necrosis is frequently observed in tuberculosis infections?
Which nutrient is predominantly found in apples?
Cell was discovered by
Under the electron microscope, which cell organelle is visible as either clusters or single, small dots that float freely in the cytoplasm?
Which phylum does "Fasciola," a type of flatworm, belong to?
What is the cause of cholera?
Which of the following disease is caused by the deficiency of iodine in food?
Which part of the human brain is located above the medulla and below the midbrain?
What is the normal RBC count in an adult male?