Which of the following shows the CORRECT expression for target mean strength ($$f_{om}$$) of concrete. if the characteristic strength and standard deviation is given by $$f_{ok}$$, and $$\sigma$$ respectively?

A

$$f_{om} = f_{ok} + 1.65 \sigma$$

B

$$f_{om} = f_{ok} - 1.65 \sigma$$

C

$$f_{om} = f_{ok} + \frac{\sigma}{1.65}$$

D

$$f_{om} = f_{ok} + 1.5 \sigma$$