True about certain lethal dose (CLD):: (A) 2 times of safely tolerated dose, (B) 4 times of safely tolerated dose, (C) 5 times of safely tolerated dose, (D) 6 times of safely tolerated dose
Answer is B. Fluoride toxicity:

Safety recommended dose — Above 3 yrs old child is 1mg/day and adult 1.7 - 3.3 = 2.2 approx.
STD (safety tolerated dose) — 8 to 16 mgF/kg wt.
CLD (certain lethal dose) — 32 to 64 mgF/kg wt (4 times of STD) = 56 g.
Probable lethal dose = 5 mg/kg wt.

Essentials of preventive and community dentistry 
Soben Peter 
5th edition