Great changes __________ in my hometown since 1990.
A:have been taken place
B:have taken place
C:take place
D:had taken place
根据拼音写出相应的词语。
如图,在四边形ABCD中,∠ADC=∠ABC=90°,AD=CD,DP⊥AB于P.若四边形ABCD的面积是18,则DP的长是( ).