643.Many people are still _________ the habit of writing silly things _____ public places.
A.at;at
B.in;in
C.for;in
D.by;for
644.He looked quite healthy ______ he was _____ the age of seventy.
A.when;at
B.because;in
C.into;of
D.during;at
643.答案是B;
be in the habit of doing sth.有干某事的习惯;in public places=in public 在公共场所。
644.答案是D;
at the age of 在…岁的时候。此句前后表示转折关系。