17. The primary purpose of the passage is to
(A) discuss a problem that hinders precise perception of the world
(B) point out the inadequacies of accepted units of measurement
(C) criticize attempts to distinguish between perceiving subjects and perceived objects
(D) compare and contrast rival scientific hypotheses about how the world should be measured and observed(A)
(E) suggest the limited function of sensory observation
18. According to the passage, quanta with an extremely long wavelength cannot be used to give complete information about the physical world because they
(A) exist independently of sense-data
(B) are graduated only in inches
(C) have an insignificant amount of energy
(D) cannot, with present-day instruments, be isolated from quanta of shorter wavelength(E)
(E) provide an insufficiently precise means of measuring length and position