Kaplan question

Andrea worked for lady Gertrude and part of her job was to buy plants, ornaments and furniture for the gardens of her county manor house. When her daughter, Harriet, was about to finish studying at uni, Lady G told Andrea that Harriet would be taking on these responsibilities.
A few weeks later, Andrea ordered some very expensive roses and marble statues from Lady G main supplier and collected the goods the next day for her own garden. The invoice was delivered to Lady G.
Which of the following best describe the agency relationship between andrea and lady Gertrude ?
A Ratification
B Express agreement
C Implied agreement
D Necessity