Echolocating bats emit sounds in patterns—characteristic of each species—that contain both frequency-modulated (FM) and constant-frequency (CF) signals. The broadband FM signals and the narrowband CF signals travel out to a target, reflect from it, and return to the hunting bat. In this process of transmission and reflection, the sounds are changed, and the changes in the echoes enable the bat to perceive features of the target.
The FM signals report information about target characteristics that modify the timing and the fine frequency structure, or spectrum, of echoes—for example, the target’s size, shape, texture, surface structure, and direction in space. Because of their narrow bandwidth, CF signals portray only the target’s presence and, in the case of some bat species, its motion relative to the bat’s. Responding to changes in the CF echo’s frequency, bats of some species correct in flight for the direction and velocity of their moving prey.
24. According to the passage, the information provided to the bat by CF echoes differs from that provided by FM echoes in which of the following ways?
(A) Only CF echoes alert the bat to moving targets.
(B) Only CF echoes identify the range of widely spaced targets.
(C) Only CF echoes report the target’s presence to the bat.
(D) In some species, CF echoes enable the bat to judge whether it is closing in on its target.（D）
(E) In some species, CF echoes enable the bat to discriminate the size of its target and the direction in which the target is moving.
25. According to the passage, the configuration of the target is reported to the echolocating bat by changes in the
(A) echo spectrum of CF signals
(B) echo spectrum of FM signals
(C) direction and velocity of the FM echoes
(D) delay between transmission and reflection of the CF signals（B）
(E) relative frequencies of the FM and the CF echoes
26. The author presents the information concerning bat sonar in a manner that could be best described as
(A) argumentative
(B) commendatory
(C) critical
(D) disbelieving（E）
(E) objective
27. Which of the following best describes the organization of the passage?
(A) A fact is stated, a process is outlined, and specific details of the process are described.
(B) A fact is stated, and examples suggesting that a distinction needs correction are considered.
(C) A fact is stated, a theory is presented to explain that fact, and additional facts are introduced to validate the theory.
(D) A fact is stated, and two theories are compared in light of their explanations of this fact.（A）
(E) A fact is stated, a process is described, and examples of still another process are illustrated in detail.
