Which of the following is true of the case of Yu Fang Tan v$.$ Arnel Management Co$.?$

a$.$ The landlord was not required to respond to the need for extra precautions because of the high additional expenses he would incur.

b$.$ There was a chain of events that put the landlord on notice that extra precautions were needed.

c$.$ Chang and Tan filed suit against Arnel Management Co$.$ because the complex gate was broken.

d$.$ All of these choices are correct.