</p> 23 Paragraph 2 .( i# p( U% L( _1 n. ~2 G4 h
24 Paragraph 3 .
2 K, j1 V9 x3 s$ a; C 25 Paragraph 4 . r1 ~1 u/ w1 z
26 Paragraph 5 .
' Q( Q: Y" f3 T! g; w1 E2 m0 M A Study of Sound
# ~. g" W4 @7 U4 d& u B Highway Police
$ F1 p" i4 n6 x% b7 A3 W& X5 E1 m5 r C Working Principles8 P& K' [6 j1 R/ U9 \
D Early Use of "Radar"
5 V) N% v$ J/ v! e7 E q' X( d+ ` E Useful Tools5 S, O0 ^# e# F" S# }, E( ]
F Discovery by Chance
! ?5 F, H" v3 p 27 Echo-sounding devices were early used to . ?& A4 |! Y M5 r. P. H
28 Ultrasonic device were used to .! {) X3 J; i9 O g
29 Police use radar on highways to . r; o S4 w+ w4 H1 V" {& Q$ W# N
30 Radar helps pilots to ." Z8 T+ k h& i. x: t
A detect nearby objects: e# {5 g) [$ b$ J) ?, X7 w1 T8 K
B determine the depth of the ocean water6 B; q9 [* C0 [1 X$ e& y3 e
C decide how fast you drive5 K" ^5 g T% p
D stop passing ships# X; Z3 M$ N( p2 H% H2 j: z: g8 S
E map the ocean floor
! a) Z$ t! X$ ~! u- ?) ] F observe water flow |