SQL & PL/SQL

Hello,

I am using Oracle 11.2, how can I concatenate the value of multiple columns into one string with one SQL:

create table testTb(classId number(5), classRoom varchar2(32));

insert into testTb value(101, 'room101');
insert into testTb value(101, 'room201');
insert into testTb value(101, 'room301');
insert into testTb value(202, 'room444');
insert into testTb value(202, 'room555');

I would like to generate the result as followings:

Class 101 is in room101, room201, room301
Class 202 is in room444, room555

Thanks,