Exotoxin is a toxin secreted by bacteria. exotoxin can cause damage to the host by destroying cells or disrupting normal cellular metabolism
The order of the answers are as follows:
B
C
D
A
The mass decay rate is of the form

where
m₀ = 3000 g,the initial mass
k = the decay constant
t = time, years.
Because the half-life is 30 years, therefore

After 60 years, the mass remaining is

Answer: 750 g