Final answer:
Equations in recursive systems can be estimated using OLS, while two-stage least squares is necessary in certain cases of endogeneity.
Step-by-step explanation:
The statement (a) Equations that are part of a recursive system can be validly estimated using OLS is true. In a recursive system, each equation depends on the values of previous equations in the system. However, if the equations are linear and each equation only depends on lagged values of variables, ordinary least squares (OLS) can be used to estimate the parameters.
On the other hand, the statement (b) Unnecessary use of two-stage least squares (2SLS) is false. Two-stage least squares (2SLS) is a method used to estimate parameters in systems of simultaneous equations when there are endogeneity issues or instrumental variables available. It is used when the OLS estimator is biased and inconsistent due to these issues, making it necessary in certain cases.